GIFT   OF 
MICHAEL  REESE 


STANDARD 
POLYPHASE  APPARATUS 

AND 

SYSTEMS 


BY 

MAURICE  A.  QUDIN,  M.  S. 

Mem.  Am.  Ins.  E.  £. 


WITH  MANY  PHOTO-REPRODUCTIONS. 
DIA  GRA  MS  A  ND  TA  BLES 


NEW   YORK 

D.   VAN    NOSTRAND    COMPANY 
23  MURRAY  AND  27  WARREN  STREETS 

LONDON 

SAMPSON    LOW,    MARSTON   &    COMPANY 

LIMITED 

&t.  Bunstan'0  ^ouse 

FETTER  LANE,  FLEET  STREET,  E.G. 

1899 


COPYRIGHT,  1899 

BY 
D.  VAN  NOSTRAND  COMPANY. 


C.    J.    PETERS    &    SON,    TYPOGRAPHERS, 
BOSTON. 

PLIMPTON    PRESS, 

H.    M.     PLIMPTON    &    CO.,    PRINTERS    &    BINDERS, 
NORWOOD,%MASS.,    U.S.A. 


PREFACE. 


THE  development  of  polyphase  apparatus  and  the  appli- 
cation of  polyphase  systems  to  the  solution  of  engineering 
problems,  have  been  so  rapid  and  varied  of  late,  that  there 
is  no  available  literature  on  the  subject  which  is  at  once 
practical  and  up-to-date.  The  excuse  for  this  little  book 
is  the  demand  for  information,  in  a  convenient  form,  on  the 
characteristics  and  uses  of  the  various  types  of  polyphase 
apparatus,  and  on  the  actual  working  of  the  several  poly- 
phase systems  now  sanctioned  by  the  best  practice. 

These  notes  are  intended  for  electrical  engineers,  cen- 
tral station  men,  and  others  who  talk  about,  operate,  or  are 
interested  in  polyphase  machinery.  While  a  certain  gene- 
ral aquaintance  with  alternating-current  apparatus  is  pre- 
supposed on  the  part  of  the  reader,  the  author  believes  that 
the  reader  whose  experience  has  been  confined  to  direct- 
current  machinery,  will,  nevertheless,  experience  no  great 
difficulty  in  reading  and  understanding  this  book. 

In  view  of  the  amazing  increase  in  number  and  magni- 
tude of  installations  for  the  transmission  of  power  by  poly- 
phase currents,  this  book  has  been  written  with  special 
reference  to  the  problems  that  belong  to  this  class  of 
engineering  work. 

The  author  desires  to  acknowledge  his  indebtedness  to 
many  electrical  manufacturing  concerns  for  the  use  of 
much  special  and  valuable  information,  and  to  the  electri- 
cal press  for  the  use  of  a  number  of  plates. 

NEW  YORK,/««*,  1899. 


CONTENTS. 

CHAPTER  PAGE 

I.  DEFINITIONS  OF  ALTERNATING-CURRENT  TERMS  .    .  i 

II.    GENERATORS 17 

III.  GENERATORS  (Concluded} 38 

IV.  INDUCTION  MOTORS 60 

V.    SYNCHRONOUS  MOTORS 92 

VI.    ROTARY  CONVERTERS 106 

VII.    STATIC  TRANSFORMERS 120 

VIII.  STATION  EQUIPMENT  AND  GENERAL  APPARATUS  .    .  140 

IX.    TWO-PHASE  SYSTEM 166 

X.    THREE-PHASE  SYSTEM 180 

XI.     MONOCYCLIC  SYSTEM 194 

XII.    CHOICE  OF  FREQUENCY 207 

XIII.  RELATIVE  WEIGHTS  OF  COPPER  FOR  VARIOUS  SYSTEMS,  214 

XIV.  CALCULATION  OF  TRANSMISSION  LINES  .  221 


(v) 


STANDARD   POLYPHASE  APPARATUS 
AND  SYSTEMS. 


CHAPTER    I. 

INTRODUCTORY. 

DEFINITIONS    OF  ALTERNATING-CURRENT 
TERMS. 

Alternating  Currents.  —  On  account  of  the  limitation 
imposed  by  the  space  of  this  book,  mathematical  demon- 
strations of  alternating-current  phenomena  have  been 
omitted  in  the  following  pages,  and  the  chapter  will  be 
found  to  consist  mainly  of  elementary  explanations  and 
statements  which  partake  of  the  nature  of  definitions.  It 
is  hoped  that  these  definitions  will  be  found  useful  in 
aiding  the  uninformed  reader  to  obtain  a  clearer  under- 
standing of  the  principles  underlying  polyphase  appa- 
ratus and  methods.  For  a  more  comprehensive  treatment 
of  alternating-current  phenomena,  the  reader  is  referred  to 
the  many  works  on  the  subject. 

The  alternating-current  generator  was  one  of  the  ear- 
liest applications  of  the  principles  of  induction.  Unlike 
the  current  from  the  direct-current  generator,  which  came 
at  a  later  date,  the  alternating  current  rapidly  changes  its 
value  and  direction,  the  fluctuations  being  periodical. 
Such  a  current  reaches  a  maximum  in  one  sense,  de- 


2  POLYPHASE   APPARATUS   AND   SYSTEMS. 

clines  to  zero,  reverses,  and  then  attains  a  maximum  in 
the  other  sense,  as  often  as  the  pressure  of  the  generator 
follows  this  variation.  This  variation  of  current,  or  of 
pressure  in  its  simplest  and  ideal  form,  follows  the  law 
of  simple  harmonic  motion,  and  may  be  represented  by 
the  projection  of  a  point  moving  in  a  circle,  with  a  con- 
stant velocity,  upon  a  perpendicular  diameter. 

The  development  of  this  motion  and  its  application  to 
the  variation  of  the  current  or  the  induced  pressure  of  an 
ideal  alternating-current  generator  is  illustrated  in  Fig.  i. 
The  point  P  on  the  circle  is  considered  as  moving  with  a 


Fig.   1. 


constant  velocity.  Its  projection  on  the  diameter  is  the 
value  of  the  pressure  at  any  instant  of  time.  The  circle 
represents  a  complete  revolution  or  cycle  of  change  of 
current  or  pressure.  The  straight  line  to  the  right  is  the 
development  of  the  circle  expressed  in  degrees,  360  of 
which  constitute  one  complete  period.  On  this  line  the 
instantaneous  values  of  the  current  or  the  pressure  derived 
from  the  projection  of  P  are  plotted.  It  is  seen  that  a 
line  drawn  through  these  points,  obtained  for  the  com- 
plete revolution,  gives  a  sine-curve. 

On  account  of  the  irregular  magnetic  field,  in  practice 
few  alternating-current  generators  give  rise   to  pressures 


ALTERNATING-CURRENT   TERMS.  3 

following  a  simple  sine-law.  The  variation  from  a  sine- 
curve  is  not  so  great,  in  the  majority  of  alternating-current 
generators,  but  that,  for  purposes  of  most  commercial  cal- 
culations, their  electro-motive  forces  can  be  considered  as 
simple  harmonic  quantities. 

The  formula  for  the  flow  of  current  in  an  alternating 
system  of  conductors  is,  in  its  general  form,  similar  to 
that  used  for  determining  the  flow  in  a  direct-current 
system.  It  differs  from  Ohm's  law  only  in  the  introduc- 
tion of  certain  factors,  which,  however,  may  become  so 
complex  as  to  conceal  the  simple  quantities  of  the  equa- 
tion resistance  and  E.M.F.  The  value  of  these  factors 
depends  on  three  well-known  properties  of  a  conductor. 
These  are : 

1.  Inductance. 

2.  Capacity. 

3.  Virtual  Resistance. 

Inductance.  —  The  magnetic  field,  surrounding  a  circuit 
through  which  a  current  is  flowing,  exerts  no  influence  on 
the  circuit  in  the  case  of  a  direct  current  of  constant  value. 
In  the  case  of  an  alternating  current  it  is  of  far  greater 
practical  importance,  and  gives  rise  to  a  variety  of  phe- 
nomena. The  magnetic  flux  then  varies  periodically  with, 
and  in  the  same  manner  as,  the  current  and  E.M.F.  The 
setting  up  of  this  magnetic  flux  —  or  lines  of  force,  as  they 
are  sometimes  called  —  produces  an  E.M.F.  in  the  circuit, 
in  opposition  to  the  induced  E.M.F.  This  counter  E.M.F., 
or  E.M.F.  of  self-induction,  is  stronger  when  the  magnetic 
flux  is  changing  most  rapidly  ;  therefore  arriving  at  a  max- 
imum, 90°,  later  than  the  flux  and  the  current  producing 
the  flux.  The  result  of  this  counter  E.M.F.  is  that,  when 


4      POLYPHASE  APPARATUS  AND  SYSTEMS. 

an  external  E.M.F.  is  applied,  the  current  does  not 
immediately  attain  its  maximum,  and,  when  the  E.M.F.  is 
withdrawn,  the  current  persists  for  awhile.  The  current 
reaches  its  maximum  later  in  point  of  time  than  the 
E.M.F., —  i.e.,  is  always  lagging  behind  the  E.M.F.  It 
would  seem  as  if  a  current  of  electricity  possessed  a 
quality  of  the  nature  of  the  inertia  of  matter. 

The  strength  of  this  flux,  or  the  induction  as  Faraday 
called  it,  is  determined  by  the  current.  The  extent  to 
which  a  gixen  flux  affects  a  circuit  in  a  non-magnetic 
medium  —  i.e.,  the  magnitude  of  the  counter  E.M.F.  — 
depends  solely  upon  the  geometry  of  the  circuit.  If  the 
circuit  is  wound  in  a  coil,  or  so  arranged  that  in  the 
periodic  variation  of  the  flux  the  same  lines  of  force 
encircle  more  than  one  portion  of  the  conductor,  the  coun- 
ter E.M.F.  will  be  increased. 

That  constant  quality  of  a  circuit  which  determines  its 
inductive  effects  is  called  inductance.  The  inductance 
may  be  either  self  or  mutual  inductance,  according  as  the 
circuit  is  isolated  or  acted  on  by  an  adjacent  circuit,  also 
carrying  a  current.  Inductance  is  frequently  called  the 
co-efficient  of  induction.  The  symbol  L  is  used  to  desig- 
nate self-inductance,  —  the  unit  of  measurement  of  which 
is  the  henry. 

Capacity.  —  Like  inductance,  the  capacity  of  a  circuit 
depends  upon  its  geometry  and  its  surroundings.  It  is 
the  quality  which  a  conductor  possesses  of  being  able  to 
hold  a  quantity  of  electricity.  A  combination  of  con- 
ductors or  conducting  surfaces,  advantageously  placed  to 
hold  the  greatest  possible  quantity  of  electricity,  is  called 
a  condenser.  All  insulated  lines  act  more  or  less  like 
condensers.  The  charging  or  discharging  current  of  a 


ALTERNATING-CURRENT   TERMS.  5 

condenser  is  greatest  when  the  rate  of  change  of  effective 
pressure  is  greatest ;  that  is,  when  the  E.M.F.  is  at  zero  at 
the  moment  of  passing  from  negative  to  positive,  or  vice 
versa.  The  effect  of  capacity,  then,  is  opposite  to  the 
effect  of  inductance,  and  may  neutralize  it,  or  even  over- 
come it,  when  existing  in  the  same  circuit.  In  a  circuit 
having  capacity,  the  current  may  lead  the  E.M.F.  in  phase. 
Fig.  2  shows  the  lead  produced  by  capacity.  The  curve 
V  represents  the  curve  of  E.M.F.,  and  /  the  current 
curve  leading  the  E.M.F.  The  unit  of  measurement  of 
capacity  is  the  farad,  and  is  usually  represented  by  the 
symbol  K. 

Impressed  E.M.F.  —  The  more  frequently  an  alternat- 
ing current   is  re- 
versed,   the    less         l/^/^^^\^\ 
time  is  there  avail-      / /  \  \ 

able  for  it  to  reach   /  / \  \ 

the  value  it  would     / 
have   irr   a   direct- 
current     system. 

-r      i    •        ,u  •  •  Fig.  2. 

To  drive  this  maxi- 
mum current  through  an  alternating  system  of.  conductors 
having  inductance,  requires  a  greater  E.M.F.  than  is  needed 
in  a  direct-current  system  to  produce  this  same  current. 
The  inductance  of  a  circuit,  as  explained,  determines  the 
counter  E.M.F. ;  and  it  must  be  overcome  by  art  added 
amount  to  the  E.M.F.  required  to  produce  the  same  cur- 
rent in  a  direct-current  system.  The  name  of  impressed 
E.M.F.  has  been  given  to  this  resultant.  The  counter 
E.M.F.  acts  at  right  angles  to  the  current,  and  is  greatest 
when  the  current  is  reversing  its  sign,  or  when  the  rate  of 
change  of  the  lines  of  force  is  greatest.  The  values  and 


6      POLYPHASE  APPARATUS  AND  SYSTEMS. 

direction  of  the  impressed  E.M.F.  and  its  components  may 
be  considered  in  a  diagram.  In  Fig.  3  the  impressed 
E.M.F.  is  shown  as  the  hypotenuse  of  a  right-angled  tri- 
angle. That  component  of  the  E.M.F.  which  would  drive 
the  same  current  through  a  circuit  without  inductance, 
being  necessarily  in  phase  with  the  current,  is  shown  as 
lagging  behind  the  impressed  E.M.F.  by  an  angle,  <l>,  and 
by  a  length  equal  to  its  magnitude.  In  quadrature  with 
the  component  is  the  E.M.F.  of  self-induction,  the  mag- 
nitude of  which  determines  the  length  of  the  line  in  the  dia- 
gram. The  magnitude  of  the 
impressed  E.M.F.  is  then  rea- 
dily  found.  The  name  of  en- 
ergy  E.M.F.  has  been  given 
to  that  component  in  phase 
with  the  current,  and  which  is 
effective  in  doing  any  work  in 


/ft  Energy  EMF  .         .  .          , ,      , 

a  circuit.     As  all  the  quanti- 
ties in  the  diagram  must  follow 

the  law  of  simple  harmonic  motion,  the  curve  of  self-induc- 
tive E.M.F.  will  be  shown  in  the  same  way  as  the  curve  of 
impressed  E.M.F.  The  effect  of  this  inductive  component, 
in  increasing  the  impressed  volts  needed  to  cause  a  given 
current  to  flow,  is  shown  in  Fig.  4.  The  curve  RI  repre- 
sents the  energy  component  of  the  impressed  E.M.F. 
which  would  drive  the  current  if  there  were  no  induc- 
tance. It  is  equal  in  value  to  the  product  of  the  current 
and  the  resistance.  In  quadrature  with  it,  is  the  inductive 
E.M.F.,  designated  by  the  curve  pLI,  p  being  equal  to 
$Ny  where  N  is  the  number  of  complete  cycles  per  second, 
and  L  the  inductance.  This  is  the  component  required  to 
offset  the  effect  of  the  inductance.  By  adding  the  ordi- 


ALTERNATING-CURRENT   TERMS. 


nates  of  the  two  curves,  we  obtain  a  third  curve,  V,  also 
following  the  sine-curve  law.     This  is  the  curve  of  the 


Pig.  4. 

impressed  E.M.F.  required  to  produce  the  given  current 
in  this  particular  circuit. 

Impedance  ;  Reactance.  —  Impedance  is  the  total  opposi- 
tion in  a  circuit  to  the  flow  of  current.  It  determines  the 
maximum  current  that  can 
flow  with  a  given  impressed 
E.M.F.  It  is  made  up  of 
a  resistance  component  and 
another  component  to  which 
the  name  of  reactance  has 
been  given.  The  relations 
of  resistance,  reactance,  and 
impedance  are  shown  in  Fig. 
5.  As  there  may  be  energy 
losses  external  to  a  circuit,  and  yet  dependent  on  that  cir- 
cuit, which  require  a  flow  of  current  that  cannot  be  deter- 
mined by  a  calculation  based  upon  the  ohmic  resistance 
alone,  it  is  not  correct  to  designate  the  resistance  com- 
ponent as  the  ohmic  resistance.  Such  losses  are  those 


R-Resistance 
Fig.  5. 


8 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


-Resistance 


'*, 


'*e0 


**& 


due  to  hysteresis  in  transformers  and  iron  cores,  which 
have  the  effect  of  a  small  transformer  interposed  in  the 
circuit.  This  component  of  impedance  is  termed  energy 
resistance,  and  the  other,  reactance  which  has  sometimes 
been  called  the  inductive  resistance. 

Reactance  is  the  effect  of  self-induction  expressed  in 
ohms.  It  becomes  prominent  in  lines  of  large  cross- 
section.  The  relative  value  of  reactance  to  resistance  can 
be  reduced  by  selecting  a  number  of  conductors  of  small 
areas  having  a  combined  equal  resistance.  For  instance, 

when  for  one  No.  ooo 
wire  two  No.  I  wires  are 
substituted,  the  resistance 
will  remain  the  same,  but 
the  reactance  will  be  al- 
most  halved. 

When  capacity  is  intro- 
duced into  the  circuit,  the 
current  may  lead  in  phase.  Fig.  6  illustrates  the  effect  of 
capacity  on  the  circuit.  The  reactance  due  to  capacity,  or 
condensance  as  it  is  designated,  acts  in  the  opposite  direc- 
tion to  the  reactance  of  inductance.  The  impedance  in 
Fig.  6  is  the  resultant  of  the  resistance  and  the  capacity  re- 
actance. When  capacity  and  inductance  are  both  present, 
the  impedance  is  the  resultant  of  the  resistance  component 
and  a  component  equal  to  the  difference  between  the  numer- 
ical values  of  the  condensance  and  reactance.  In  Fig.  7 
,  the  magnetic  reactance  is  laid  off  above  the  line  of  resis- 
;  tance  and  in  quadrature  with  it.  The  capacity  reactance,  or 
condensance,  is  represented  as  having  a  greater  numerical 
value,  and  acting  in  opposing  direction.  The  resultant  im- 
pedance is  readily  found.  When  the  inductance  is  equal 


ALTERNATING-CURRENT   TERMS.  9 

to  the  capacity,  the  current  is  in  phase  with  the  impressed 
volts,  and  follows  Ohm's  law. 

In  aerial  conductors  of  low  resistance,  the  reactance  is 
often  prominent,  and  the  distribution  of  E.M.F.  may  be 
seriously  affected  by  it.  It  becomes  important,  then,  in 
selecting  conductors  for  transmission  lines,  that  those  of 
large  cross-section,  and  correspondingly  low  resistance,  be 
avoided  as  much  as  possible,  except  in  special  cases,  as, 
for  instance,  in  the  employment  of  rotary  converters  sup- 
plied by  its  own  set  of  con- 
ductors, where  some  react- 
ance is  desirable. 

Virtual  Resistance.  —  If 
the  cross-section  of  a  con- 
ductor carrying  an  alter- 
nating current  is  resolved 
into  many  elements,  it  will 

be  seen  that  the  internal  portions  are  subject  to  greater 
inductive  effects  than  the  elements  nearer  the  surface. 
The  outer  streams  of  current  suffer  less  opposition,  and 
reach  a  maximum  sooner  than  those  centrally  located. 
In  large  conductors,  carrying  heavy  currents  of  high  fre- 
quency, there  may  not  only  be  no  current  flowing  in  the 
central  portion  of  the  conductor,  but  a  condition  may  exist 
where  a  current  will  flow  in  the  opposite  direction.  The 
central  core  is  then  not  only  valueless  as  a  conductor,  but 
had  better  be  omitted. 

As  a  result  of  the  reduction  of  the  effective  cross-section 
of  a  conductor  carrying  an  alternating  current,  the  resis- 
tance is  increased,  and  slightly  less  current  will  flow  than 
would  if  the  specific  resistance  and  the  inductance  of  the 
wire  are  alone  considered.  This  increment  of  resistance 


10        POLYPHASE  APPARATUS  SYSTEMS. 

of  a  conductor  is  called  its  virtual  resistance.  The  phe- 
nomenon is  also  called  the  skin  effect. 

The  best  shape  for  conductors  of  large  cross-section, 
carrying  heavy  alternating  currents,  is  that  of  a  tube  or 
flat  strip. 

In  common  practice  the  sizes  of  wire  and  the  rapidity 
of  current  reversals  are  not  such  as  to  appreciably  produce 
this  effect.  The  ratio  of  the  resistance  of  a  conductor 
carrying  an  alternating  current,  to  its  resistance  when  a 
direct  current  is  flowing,  can  be  readily  computed  for  dif- 
ferent sizes  of  conductors  and  reversals  of  current. 

In  Fig.  8  the  ordinates  represent  the  product  of  area 
and  cycles  per  second.  Corresponding  factors  for  the 
virtual  or  apparent  resistance  of  cylindrical  copper  con- 
ductors are  read  off  the  horizontal  scale.  The  factor  for 
a  conductor  of  any  other  non-magnetic  metal  will  vary 
in  the  ratio  of  its  conductivity  to  that  of  copper.  In  the 
case  of  magnetic  materials,  especially  iron,  the  factor  for 
the  virtual  resistance  is  greater  than  that  in  the  curve. 

Energy  in  a  Circuit.  —  The  work  done  in  a  circuit  will 
always  be  some  product  of  the  current  and  the  quantities 
in  phase  with  it.  In  a  direct-current  system  the  product 
of  measured  volts  and  amperes  will  give  the  energy  of  the 
circuit.  In  an  alternating-current  system  the  product  of 
the  measured  amperes,  and  the  component  of  impressed 
E.M.F.  in  phase  with  the  current,  —  i.e.,  the  energy 
E.M.F.)  —  will  give  the  energy.  The  component  of  E.M.F. 
in  quadrature  with  the  current  —  i.e.,  the  induction  com- 
ponent —  drives  a  wattless  current,  and  consequently  adds 
nothing  to  the  energy  of  the  circuit.  The  product  of  the 
impressed  E.M.F.  and  the  current  will  be  easily  under- 
stood to  give  too  large  results.  This  product  is  the 


ALTERNATING-CURRENT   TERMS. 


II 


apparent  watts  of  the  circuit.  The  error  in  calculating 
the  power  by  the  measured  amperes  and  volts  will  depend 
upon  the  extent  of  the  displacement  in  phase  of  the  im- 
pressed E.M.F.  and  the  current,  or  the  angle  of  the  lag, 
usually  denoted  as  <l>.  The  energy  in  the  circuit  can  be 


85 


75 


£®\ 


•6  *> 


100      1.04      1.08      l.fc       1.16        1.2       1.22      124-      1.28 
Factor  -for  Virtual  Resistance. 
Fig.  8. 

found  by  multiplying  the  product  of  volts  and  amperes  by 
the  cosine  of  this  angle  of  lag. 

Power  Factor  —  Induction  Factor.  —  The  ratio  of  the 
true  watts  in  the  circuit,  as  measured  by  an  indicating 
wattmeter,  to  the  apparent  watts,  — the  volt-amperes,  —  is 
called  the  power  factor.  The  power  factor  is  useful  in 
determining  the  true  energy  in  a  circuit  when  the  apparent 


12        POLYPHASE  APPARATUS  SYSTEMS 

energy  is  known,  the  resistance  when  the  impedance  is 
known,  the  energy  volts  when  the  total  impressed  volts 
are  given,  and  the  energy  current  when  the  total  current 
is  known. 

The  quantities  in  quadrature  with  the  energy  values  of 
current  and  E.M.F.,  and  with  the  resistance,  may  be  deter- 
mined in  the  same  way,  from  the  resultants  by  a  multiplier 
or  factor,  called  the  induction  factor.  As  the  power  factor 
is  proportional  to  the  energy  quantities,  and  the  induction 
factor  to  the  components  in  quadrature  with  them,  it  fol- 
lows that  the  former  must  be  numerically  equal  to  the 
cosine,  and  the  latter  to  the  sine  of  the  lag  angle.  Ac- 
cordingly, a  table  of  cosines  and  sines  for  all  angles  will 
give  the  corresponding  power  and  induction  factors. 

Wattless  Current. — The  component  of  the  total  current 
in  quadrature  with  the  energy  current  is  called  the  watt- 
less current.  It  should  be  understood  that  the  current 
and  other  quantities  of  a  circuit  are  resolved  into  compo- 
nents for  the  sake  of  a  better  understanding  of  the  phe- 
nomena taking  place  in  '•he  circuit.  There  is  actually  but 
one  current  flowing,  as  there  is  but  one  E.M.F.,  in  any  one 
part  of  a  circuit.  The  presence  of  reactance,  either  in  the 
transmission  circuit  or  in  the  apparatus  connected  to  it, 
increases  the  lag-angle,  and  consequently  the  wattless 
current.  This  component  does  no  work  in  a  circuit,  but 
increases  the  total  current,  and  thereby  the  heating  of 
conductors.  The  wattless  current  required  to  balance  the 
reactance  may  become  sufficiently  great  to  practically  tax 
the  full  capacity  of  generators  and  of  conductors,  although 
very  little  energy  is  being  generated  or  transmitted.  If  it 
were  possible  to  have  conductors  without  resistance,  a 
true  wattless  current  could  then,  in  fact,  actually  exist  in 


ALTERNATING-CURRENT   TERMS.  13 

an  alternating-current  circuit.  In  such  a  case  the  current 
would  be  in  quadrature  with  the  impressed  E.M.F.,  and 
the  circuit  would  give  back  as  jnuch  energy  as  it  received, 
the  sum  being  zero. 

Relative  Values.  —  Designating  the  current  as  /,  resis- 
tance as  R,  reactance  as  S,  and  impedance  as  U,  from 
what  has  preceded,  the  following  relations  will  be  under- 
stood : 

1.  The  reactance  ")         Induction  E.M.F.  consumed  in  line 

of  a  line,  S,    )  / 

m,      .  TT       Impressed  E.M.F.  consumed  in  line 

2.  The  impedance,  £7,  =  —          

3.  The  energy  component  of  E.M.F.  consumed  by  the  resis- 

tance, R,  of  a  conductor  is  IR,  and  is  in  phase  with  the 
current. 

4.  The   inductive    component   of    E.M.F.   consumed   by   the 

reactance,  S,  of  a  conductor  is  fS,  and  is  in  quadrature 
with  the  current. 

5.  The  impressed  E.M.F.  consumed  by  the  impedance,  £7,  of  a 

conductor,  is  IU. 

5.   The  energy  loss  in  a  conductor  is  1*R,  and  depends  on  the 
current  and  resistance  only. 

Voltage  Drop  Due  to  Power  Factor.  —  The  E.M.F.  con- 
sumed by  the  impedance,  IU,  does  not  represent  the  vol- 
tage drop  in  a  conductor,  as  it  is  usually  out  of  phase  with 
the  impressed  E.M.F.  as  well  as  with  the  current.  This 
voltage  drop,  as  will  be  shown,  can  be  anything  between 
IR  and  IU.  It  will  depend  upon  the  difference  in  phase 
between  the  current  and  the  impressed  E.M.F.,  or  the 
lag  angle,  and  can  be  easily  determined  when  the  power 
factor  is  known.  In  Figs.  9  to  14,  let  OE  be  the  E.M.F. 
at  the  receiving  end  of  a  transmission  line.  For  various 


POLYPHASE  APPARATUS  SYSTEMS. 


power  factors  at  the  receiving  end  of  the  line,  there  will  be 
corresponding  phase  differences,  <£.  Let  OI  be  the  cur- 
rent out  of  phase  with  the 
E.M.F.  by  $.  'IU,  IR, 
and  IS  have  the  relations 
heretofore  assigned  to 
them,  IR  being  in  phase 
with  (97,  and  IS  in  quad- 

S 


£=90° 


Fig.  9. 


rature  with  OI.  Where 
these  quantities  are  small 
relatively  to  the  impressed 
E.M.F,  as  they  usually 
are  in  practice,  the  drop 

of  voltage  is  IA,  equal  to  OE  —  OE',  OE  being  equal  to 
the  generator  voltage,  A  the  apparent  resistance  of  the  line. 
Assume  a  given  E.M.F.  at  the  end  of  the  line,  and  a 
constant  resistance  and  reac- 
tance. If  the  phase  displace- 
ment 4>,  or  what  is  the  same, 
if  the  power  factor  of  the 
receiving  system,  is  varied, 
the  triangle  of  electromotive 
forces  will  revolve  around  E 
as  center.  The  projection  of  A  <j>=6o° 
IU,  or  its  components,  upon 
the  E.M.F.  will  give  the  vol- 
tage drop.  With  a  lag  angle 
of  90°  (Fig.  9),  the  drop  of  voltage  is  due  to  the  reac- 
tance alone.  As  the  lag  angle  decreases,  the  drop  IA  be- 
comes less  than  the  impressed  E.M.F.  consumed  in  the  line 
IU  until  it  reaches  60°  (Fig.  10),  when  with  the  given 
values  of  IU  and  75  the  drop  is  seen  to  be  equal  to  the 


E'     U 


Pig.  10. 


ALTERNATING-CURRENT   TERMS 


impedance  1U,  and  has  the  greatest  value  it  can  have. 
As  the  phase  displacement  grows  less,  the  effect  of  the 
reactance  decreases  until  $  =  o  (Fig.  n),  when  the  drop 
is  due  to  resistance  alone,  a  case  of  a  non-inductive 
load. 


If,  now,  capacity  is  in-  6-0° 


Fig.  11. 


troduced   into  the   line 
by  the  use  of  long  cables, 
synchronous  motors,  ro- 
tary converters,  or  con- 
densers, the  phase  displacement  <£  becomes  negative.     Up 
to  30°  the  projection  of  the  reactance  is  in  opposition  to 
the  projection  of  the  impedance,  i.e.,  negative  (Fig.  12), 

and  as  a  result  the 
drop  I  A  is  less  than 
the  resistance  drop. 
Finally,  at  30°  (Fig. 

Fig.  12.      ^<^E  13)  there  is  no  drop 

of  voltage  in  the 
line ;  for  the  reactance  raises  the  voltage  as  much  as  the 
resistance  lowers  it,  and  the  line  apparently  has  no  re- 
sistance. As  the  phase  displacement  increases,  the  vol- 
tage at  the  receiving  . 
end  becomes  higher 
than  the  generator 
E.M.F.,  due  to  the 


Fig.  13 


predominating  effect 
of  the  capacity  reac- 
tance over  the  resis- 
tance. This  is  the  greatest  at  90°  (Fig.  14).  For  the 
sake  of  simplicity  we  have  assumed  in  the  foregoing  that 
the  projection  of  E  determines  the  apparent  resistance. 


i6 


POLYPHASE   APPARATUS    AND   SYSTEMS. 


0—90° 


This  is  not   strictly  accurate,  but   in  practice  the   error 
involved  will  be  found  to  be  insignificant. 

Frequency.  — The  number  of  complete  reversals  of  .alter- 
nating quantities  in  any  given  time  is  called  their  fre- 
quency. Each  complete  reversal  is  a  period  or  cycle,  and 
is  measured  in  degrees.  An  alternation  is  a  half  period 
or  cycle,  and  in  the  curve  of  impressed  E.M.F.  (Fig.  i)  is 

measured  by  the  value 
A  I  „  of  the  E.M.F.  from  o° 
to  1 80°,  and  from  180° 
to  360°.  In  a  bipolar 
generator  every  revo- 
lution of  the  armature 
corresponds  to  one 
cycle.  In  multipolar 
generators  there  will  be  as  many  cycles  for  every  revo- 
lution as  there  are  pairs  of  poles.  Frequency  is  usually 
denoted  in  cycles  per  second.  In  a  twenty-four  polar 
generator  of  300  R.P.M.,  the  number  of  alternations  per 
minute  is  7,200.  The  number  of  cycles  per  minute  is 
one-half  of  this,  or  3,600,  and  in  one  second  is  60.  Ap- 
plying this,  - 

Frequency,  or   |        Poles  X  R.P.M. 
Cycles  per  sec.  )  "  60  X  2 


Fig.  14. 


GENERATORS.  I/ 


CHAPTER  II. 
GENERATORS. 

Elementary  Forms.  —  The  simplest  form  of  polyphase 
generator  consists  of  two  single-phase  alternators  coupled 
together  on  one  shaft  in  such  a  manner  that  the  electro- 
motive forces  at  the  terminals  of  the  armature  conductors 
arrive  at  a  maximum  90°,  or  one-fourth  of  a  period,  apart. 
The  currents  from  this  machine  will  therefore  have  a  two- 
phase  relationship.  An  arrangement  of  three  such  arma- 
tures, with  similar  coils  one-third  of  a  pole  arc,  or  60 


Fig.   15. 

electrical  degrees,  apart,  will  generate  three-phase  currents. 
Fig.  1 5  illustrates  the  armature  connections  of  an  ideal 
three-phase  unit,  made  up  of  three  single-phase  alternators 
arranged  in  this  manner. 

This  combination  of  two  or  more  independent  alterna- 
tors, forming  one  polyphase  unit,  facilitates  the  regulation 
of  the  circuits  in  case  of  unbalancing,  as  the  fields  ( not 
shown  in  this  diagram )  are  separate.  This  form  of  gen- 
erator is  not  commercially  manufactured,  as  it  is  naturally 
expensive.  Being  made  up  of  smaller  machines,  the  cost 


1 8     POLYPHASE  APPARATUS  AND  SYSTEMS. 

would  be  greater  than  that  of  a  single  unit  of  the  same 
output.  Polyphase  generators  are  smaller,  and  conse- 
quently cheaper  to  build,  than  single-phase  alternators  of 
the  same  capacity.  Most  types  of  polyphase  generators 
have  one  field  and  one  armature,  with  as  many  sets  of  wind- 
ings as  there  are  phases.  Irregularities  in  the  voltage  of 
the  different  phases  —  if  any  exist  —  must  be  overcome 
in  some  other  manner  than  by  a  variation  of  the  field 
strength.  In  some  inductor  types  of  generators,  this  reg- 
ulation is  obtained  by  varying  the  number  of  armature 
turns  in  the  unbalanced  phase. 

The  principles  of  construction  and  operation  of  single- 
phase  generators  apply  equally  well  to  polyphase  machines. 
The  requirements  of  recent  alternating-current  practice, 
involving  the  transmission  and  utilization  of  power  to  an 
extent  that  is  already  beginning  to  overshadow  the  purely 
lighting  branch  of  the  art,  have  necessitated  vast  improve- 
ments in  machinery  and  methods.  Not  the  least  improve- 
ment has  been  in  polyphase  generators. 

Revolving  Armature  Type.  —  The  type  of  alternating- 
current  generator  most  commonly  employed  in  the  United 
States  is  that  in  which  the  armature  is  the  moving  mem- 
ber. For  some  time  past  in  Europe,  and  more  recently  in 
this  country,  another  type  has  been  in  use,  in  which  the 
armature  is  stationary,  and  the  field  structure  is  the  revolv- 
ing part.  All  modern  generators,  including  what  is  known 
as  the  inductor  generator,  are  modifications  of  either  the 
revolving  armature  or  the  stationary  armature  type. 

Fig.  1 6  illustrates  a  standard  form  of  belt-driven  gen- 
erator of  the  revolving  armature  construction,  designed  by 
the  Westinghouse  Electric  and  Manufacturing  Company. 
The  frame  has  graceful,  dignified  lines,  the  bearings  form- 


GENERATORS. 


ing  one  casting  with  the  lower  yoke  and  base.     The  pole- 
pieces  project  inwardly  from  the  frame,  and  are  made  up 


Fig.    16. 


of  steel  laminations  cast  into  the  yoke.     The  field  coils  are 
wound  upon  insulated  spools,  and  are  removable.     When 


2O 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


these  generators  are  built  for  automatic  compounding,  two 
field  windings,  one  for  the  separate  and  one  for  the  self- 
exciting  current,  are  required.  The  armature  is  of  the  iron- 
clad type,  and  is  built  up  of  laminations,  slotted  to  admit 
the  coils.  These  are  usually  machine-wound,  and  held  firm- 
ly in  place  by  seasoned  wedges  of  wood.  This  armature 
winding  construction,  and  the  arc  and  shields  of  an  arma- 


Fig.    17. 

ture  of  the  General  Electric  make,  are  shown  in  Fig.  17. 
An  injury  to  the  insulation,  from  lightning  or  other  causes, 
is  usually  limited  to  one  or  a  few  adjacent  coils,  \vhich  can 
be  easily  replaced  without  disturbing  the  rest  of  the  wind- 
ing. All  the  standard  belted  polyphase  generators  of  the 
revolving  armature  type  conform  to  the  general  lines  of 
the  generator  shown  in  Fig.  16.  Generators  of  an  output 


GENERATORS.  21 

greater  than  200  K.W.  are  usually  provided  with  a  third, 
or  outboard,  bearing  to  sustain  the  weight  of  the  pulley 
and  strain  of  the  belt.  Generators  of  500  K.W.,  and  over, 
are  almost  invariably  built  for  direct  connection  to  either 
engine  or  water-wheel.  If  built  for  connection  to  the 
former,  the  base  is  ordinarily  omitted,  while  generators  for 
coupling  to  water-wheels  are  provided  with  base  and  two 
bearings,  and  in  small  sizes  are  self-contained  as  a  rule, 
the  base  and  two  bearings  comprising  one  casting.  When- 


Fig.    18. 

ever  possible,  a  generator,  irrespective  of  its  size,  should 
be  direct-connected,  on  account  of  saving  of  space  and 
of  belt  losses. 

Revolving  Field  Type. —  The  revolving  field  type  of 
generator  is  one  of  a  number  of  forms  of  the  stationary 
armature  machine.  The  rotating  member,  or  field,  consists 
of  a  heavy  cast-steel  wheel,  into  which  are  bolted  or  keyed 
pole-pieces,  projecting  radially  outward.  These  are  usually 
built  up  of  laminated  sheet-iron.  Fig.  18  illustrates  a  ro- 
tating field  magnet  of  a  200  K.W.  generator  made  by  the 


22 


POLYPHASE    APPARATUS    AND    SYSTEMS. 


Walker  Company.  The  laminated  construction  prevents 
formation  of  eddy  currents,  which  would  occur  if  the  pole- 
pieces  were  solid  castings.  The  coils  are  wound  on  spools, 
placed  on  the  poles,  and  held  in  place  by  the  pole-tips. 
The  field  coils  on  large  machines  are  made  of  a  single  spiral 
of  strip-copper,  wound  on  edge.  Fig.  19  shows  the  con- 
struction of  the  field-spools  of  a  750  K.W.  General  Electric 
Company's  generator.  On  small  machines  wire  is  used. 
The  revolving  field  acts  like  a  fan,  forcing  the  air  out- 


Fig.  19. 

wardly  through  the  openings  between  the  armature  lami- 
nations. The  shields  of  the  circular  armature  structure 
prevent  undue  loss  through  windage  of  the  revolving 
field.  Direct  current  for  excitation  is  carried  to  the  field 
by  means  of  a  small  two-ring  cast-iron  or  copper  collector, 
equipped  with  carbon  brushes,  requiring  practically  no 
attention  in  operation. 

The  stationary  armature  consists  of  a  circular  cast-iron 
frame  or  spider,  inside  of  which  are  dove-tailed  sheet- 
iron  disks,  with  slots  to  receive  the  coils.  Ventilating 


GENERATORS. 


spaces  are  left  between  laminations,  through  which  the 
air  flows  rapidly  when  the  generator  is  running.  Fig.  20 
shows  the  construction  of  a  stationary  three-phase  arma- 
ture of  750  K.W.  capacity. 


Fig.  20. 


Fig.  21  is  a  sectional  view  of  the  field  and  armature 
of  a  typical  revolving  field  three-phase  generator.  The 
relation  of  the  magnetic  circuit  to  the  armature  coils  is 
clearly  shown. 


24     POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  generator  shown  in  Fig.  22  is  one  of  a  number 
installed  near  Redlands,  Cal.  It  has  20  poles,  and  runs 
at  300  R.P.M.,  giving  a  current  of  50  cycles.  The  driv- 
ing-power of  each  generator  is  supplied  by  Pelton  water- 
wheels,  keyed  to  the  shaft,  and  mounted  and  housed  on 
the  generator  base,  as  indicated.  The  machine  is  wound 
for  750  volts,  no  load,  and  generates  in  each  branch 
525  amperes.  The  commercial  efficiency  at  full  load  is 


Fig.  21. 

95.6  per  cent.  The  regulation  on  non-inductive  load  is 
7.1  per  cent.  The  cut  shows  the  armature,  slid  along 
on  its  base  to  permit  ready  inspection  of  the  field  and 
other  parts. 

Fig.  23  shows  a  Ganz  &  Co.  80  K.W.  revolving  field 
generator.  The  armature  winding  used  in  this  machine 
is  in  the  form  of  spools  bolted  to  the  outer  ring.  This 
arrangement  has  the  advantage  of  accessibility  for  inspec- 
tion and  repair.  The  field  construction  is  practically  the 


GENERATORS. 

' 


26 


POLYPHASE   APPARATUS    AND    SYSTEMS. 


same  as  that  of  the  generator  described  above.  It  is 
claimed  that  these  machines  have  a  moderate  armature 
reaction,  and  at  the  same  time,  when  short-circuited,  will 


Fig.  23. 

not  deliver  more  than  two  and  one-half  times  the  normal 
full-load  current. 

Another  form  of  the  stationary  armature  type  of  gen- 
erator is  one  in  which  the  field  winding  is  a  single  coil. 
The  exciting  coil  is  wound  on  a  bobbin,  occupying  a 


OF    THK 

UNIVERSITY 


GENERATORS.  27 

channel  on  the  periphery  of  a  cast-iron  wheel.  Two  steel 
rims  are  bolted  to  this,  the  laminations  being  formed 
into  poles.  This  is  one  of  the  original  forms  of  polyphase 
generators  ;  and  this  construction,  which  has  considerable 
merit,  was  adopted,  in  the  early  days  of  power  transmission 
apparatus,  by  European  manufacturers. 

Inductor  Type.  —  Another  modification  of  the  stationary 
armature  type  is  the  inductor  machine,  manufactured  to 
some  extent  abroad  by  Mordey,  Thury,  and  the  Allgemeine 
Elektricitats  Gesellschaft,  and  in  this  country  chiefly  by 
the  Stanley  Electric  Company.  The  distinguishing  char- 
acteristic of  this  type  is,  that  any  one  set  of  armature  coils, 
or  portion  of  the  armature  conductors,  is  subjected  to  a 
magnetic  flux  of  one  polarity  only.  The  magnetism  fluc- 
tuates from  zero  to  maximum,  and  back  again,  and  does 
not  reverse  its  sign.  Most  generators  of  this  type  have 
both  fixed  armature  and  fixed  field-windings,  the  only 
moving  part  being  the  inductor,  —  a  laminated  iron  core, 
with  polar  projections.  The  exciting  winding,  wound  into 
an  annular  coil,  is  sometimes  placed  centrally  on  the 
internal  surface  of  the  armature  spicier,  embracing  the 
revolving  element,  as  in  the  Stanley  machine.  This  is 
usually  a  ring  of  iron,  with  a  double  row  of  laminated 
polar  projections.  In  some  machines,  notably  those  made 
by  Thury  abroad  and  by  the  Warren  Company  of  San- 
dusky,  Ohio,  and  the  Westinghouse  Company,  the  arma- 
ture has  a  single  set  of  coils,  and  the  inductor  is  provided 
with  a  single  row  of  laminations.  The  annular  exciting 
coil  may  be  part  of  the  revolving  element,  and  revolve 
with  it. 

Reference  to  Fig.  24  will  show  the  general  arrangement 
of  the  magnetic  circuit  of  the  Stanley  inductor  generator. 


POLYPHASE    APPARATUS    AND    SYSTEMS. 


GENERATORS.  29 

The  annular  field  coil,  F,  is  surrounded  by  the  magnetic  cir- 
cuit, made  up  of  the  laminated  cores  AA,  the  armature 
yoke  Yv  and  the  laminated  poles  N  and  S,  and  the  field 
yoke  F2.  The  armature  windings,  consisting  of  two  com- 
plete sets,  are  laid  in  grooves  in  the  armature  cores  in  a 
manner  similar  to  the  revolving  field-machine.  It  will  be 
seen  that  the  North  and  South  poles  do  not  alternate,  but 
the  magnetic  flux  simply  pulsates  in  one  direction.  Only 
one-half  of  each  turn  of  the  armature  winding  is  in  an 
active  field  at  one  time,  the  other  half  of  the  coil  being 
between  the  poles  in  an  inactive  field.  The  E.M.F.  gene- 
rated is  one-half  as  great  as  it  would  be  if  the  polarity 
of  the  flux  were  reversed.  In  order  to  obtain  a  given 
E.M.F.  with  the  inductor  type  of  machine,  either  the  arma- 
ture windings  or  the  total  magnetic  flux  must  be  doubled. 
The  essential  characteristics,  therefore,  of  an  inductor 
generator  are  a  rather  high  density  of  the  magnetic 
circuit,  and  a  short  air  gap,  the  latter  in  order  to  reduce 
the  magnetic  leakage  to  a  minimum.  The  stationary  ele- 
ment of  the  Stanley  inductor  machine  consists  of  two 
series  windings,  forming  two  separate  armatures.  The 
currents  in  the  coils  are  usually  in  quadrature  with  each 
other,  thus  giving  a  two-phase  current.  A  three-phase 
relationship  can  be  established  by  means  of  a  symmetrical 
three-phase  winding,  or  by  making  one  set  of  coils  with 
.86  the  number  of  turns  of  the  other,  and  connecting  the 
end  to  the  middle  of  the  larger  coil.  By  the  theory  of 
the  resultant  of  electromotive  forces,  the  currents  in  the 
three  circuits  will  be  equal,  and  the  impulses  will  follow 
one  another  at  intervals  of  60°.  Fig.  25  shows  a  600- 
K.W.  Stanley  inductor  generator. 

Fig.  26  shows  a  sectional  view  along  the  shaft  of  an 


30  POLYPHASE    APPARATUS   AND    SYSTEMS. 

inductor  generator  manufactured  by  the  Warren  Electric 
Manufacturing  Company.      GG  is  the  frame,  or  spider,  of 


the    stationary   armature,   into   which    are   dovetailed    the 
laminated    polar    projections  A  A.      CC  are    the  armature 


GENERATORS.  3 1 

coils  surrounding  the  poles.  The  revolving  element  is 
made  up  of  the  spider  H  carrying  the  laminated  polar  pro- 
jections DD.  F  is  a  single  magnetizing  coil.  The  mag- 
netic circuit  is  from  G,  through  A,  to  D,  and  thence  from 
H  to  G.  It  will  be  seen  that  there  are  two  air  gaps,  one 
between  A  and  Dy  and  the  other  between  G  and  H.  As 
in  all  inductor  generators,  the  magnetism  pulsates  only, 
and  the  revolving  polar  projections  have  one  polarity. 


Fig.  26. 

The  armature  of  the  inductor  machine  made  by  the 
Westinghouse  Company  is  illustrated  in  Fig.  27.  This  is 
a  150  K.W.  generator,  designed  for  belt-driving.  The 
larger  machines  of  this  type  have  much  the  same  general 
appearance  as  the  revolving  field  machines. 

The  type  of  generator,  with  revolving  armature,  is  par- 
ticularly desirable  for  general  power  and  lighting  distribu- 
tion where  only  a  moderate  voltage  is  required.  Machines 


32  POLYPHASE   APPARATUS   AND    SYSTEMS. 

of  this  type  are  cheap  to  build.  They  can  be  automat- 
ically compounded,  without  any  complication  of  parts, 
which  is  not  the  case  in  the  revolving  field  or  inductor  gen- 
erator. 


Fig.   27. 


This  construction  is  not  suitable  for  high  or  for  low  vol- 
tages, on  account  of  the  difficulties  of  insulating  the  collec- 


GENERATORS.  33 

tor  rings  in  the  first  case,  and  of  collecting  a  large  current 
in  the  second  case. 

The  stationary  armature  can  be  easily  insulated  to  with- 
stand a  testing  pressure  of  25,000  volts  ;  and,  as  no  collect- 
ing device  is  required,  currents  of  any  volume  can  be 
cared  for. 

Generators  with  stationary  armatures  are  now  wound 
for  pressures  up  to  12,000  volts. 

Armature  Windings.  —  The  armature  windings  of  modern 
alternators  are  laid  in  slots  or  grooves,  below  the  surface 
of  the  armature  punchings.  The  shape  and  number  of  the 
slots  have  a  material  effect  upon  the  performance  of  a  gen- 
erator, as  we  will  proceed  to  show.  The  old-fashioned  iron- 
clad armature  had  one 
coil  for  each  pole,  or  pair 
of  poles,  laid  in  deep 
slots.  On  account  of  this 
grouping  of  the  conduc- 
tors into  a  coil  of  many 
turns,  this  generator  pos- 
sessed great  armature 
reaction,  and  could  be 

Fig.  28. 

short-circuited  with    no 

bad  effects.  This  construction  is  sometimes  carried  out  in 
those  modern  polyphase  generators  whose  armatures  have 
one  slot  for  each  phase  and  each  pole,  and  are  called  unitooth 
machines.  Thus  the  armature  of  an  eight-pole  two-phase 
generator  has  8  coils ;  a  three-phase  generator  of  the  same 
number  of  poles  has  1 2  groups  of  conductors.  The  shape 
of  the  armature  punchings  of  a  12-pole  unitooth  three- 
phase  generator  is  shown  in  Fig.  28.  The  advantage  of 
safety,  in  case  of  short  circuits,  is  a  doubtful  one,  as  most 


34    POLYPHASE  APPARATUS  AND  SYSTEMS. 

plants  are  provided  with  protective  devices  which  render 
a  short  circuit  more  inconvenient  than  dangerous.  Arma- 
ture reaction  deforming  the  wave-shape  of  the  E.M.F., 
and  high  inductance,  requiring  large  exciting  currents  at  full 
load,  are  often  characteristic  of  the  unitooth  winding.  As 
will  be  shown,  these  generators  can  be  designed  so  as  in 
a  great  measure  to  overcome  these  objections. 

Many  modern  polyphase  armatures  have  two  or  three 
slots  per  pole  per  phase.  The  slots  are  open,  which,  with 
the  distributed  form  of  winding,  gives  a  very  low  induc- 
tance (Fig.  29).  This  necessitates  only  a  slight  increase 
of  exciting  current  at  full  load.  Generators  with  multi- 
tooth  armatures  are 
built,  for  the  most  part, 
for  low  potentials  and  for 
low  frequencies.  They 
are  most  suitable  for 
long-distance  transmis- 
sion, where  step-up 

transformers    are    em- 
Fig.  29. 

ployed.     The  regulation 

is  good,  and  the  wave-shape  approaches   a   sine-curve,  — 
the  best  shape  for  this  work,  as  it  reduces  the  possibil- 
ity of  resonance,  or  rise  of  voltage,  at  a  distant  point  in 
the  transmission  circuit,  above  that  at  the  generating  end. 

The  various  connections  of  generator  armature  windings 
will  be  found  explained  in  chapters  on  polyphase  systems. 

Electromotive  Force.  —  The  drop  at  the  terminals  of  a 
direct-current  generator,  as  the  output  is  increased,  is  prin- 
cipally due  to  the  armature  resistance  and  reaction.  In  al- 
ternators the  IR  drop  is  generally  not  so  prominent  as  that 


GENERATORS. 


35 


due  to  inductance  and  to  armature  reaction.  The  counter 
E.M.F .  of  self-induction  lowers  the  terminal  pressure,  and 
armature  reaction  by  opposing  its  flux  to  the  field  magnet- 
ism reduces  the  effective  number  of  lines  of  force,  passing 
through  the  armature  conductors,  with  the  like  result. 

The  inductance  of  unitooth  armatures  can  be  lessened 
by  widening  the  opening  of  the  slots,  which,  at  the  same 
time,  increases  the  resistance  to  the  magnetic  flux,  —  i.e., 
the  reluctance  of  the  air  gap.  As  inductance  varies,  di- 
rectly, with  the  square  of  the  number  of  turns,  by  using 
fewer  turns  per  slot  and  more  slots, — in  other  words,  the 
distributed  form  of  winding,  —  this  disagreeable  property 
can  be  much  reduced,  without  sacrificing  efficiency,  or 
increasing  the  cost  of  the  generator. 

Armature    reac- 


tion is  greatest 
when  the  load  is 
inductive,  as  then 
the  curre*nt  lags 
behind  the  E.M.F., 
and  brings  themax- 

r  ig.    JO. 

imum     armature 

magnetism  in  the  most  favorable  position  for  demagnetiz- 
ing the  field.  The  distributed  winding  minimizes  the  evil 
effect  of  a  lagging  current.  As  armature  reaction  produces 
a  distortion  of  the  field,  a  curve  of  E.M.F.,  that  may  be  a 
sine-curve  at  no  load,  will  often  depart  widely  from  this 
form  when  the  generator  is  loaded.  The  distortion  of 
the  wave-shape  in  unitooth  machines  may  be  overcome, 
in  great  part,  by  careful  shaping  of  the  pole  pieces. 

While  the  armature  reaction,  due  to  a  lagging  current, 
lowers  the  terminal  E.M.F.  of  a  generator,  a  leading  cur- 


POLYPHASE   APPARATUS    AND    SYSTEMS. 


rent  may  have  the  opposite  effect,  by  adding  its  flux  to 
that  of  the  field. 


-\ 


\ 


\ 


A 


Fig.   31. 

The  relation  between  the  induced  E.M.F.  in  the  arma- 
ture windings  and  the  terminal  E.M.F.   of  a  three-phase 


Fig.  32. 


machine  is  shown  in  Fig.  30.     Curves  A  and  B  represent 
the  voltages  measured  between  the  common  center  and  end 


GENERATORS.  37 

of  the  armature  coils.  Curve  C,  formed  by  uniting  these 
F  electro-motive  forces,  is  the  A  E  M.F.  or  pressure  be- 
tween the  terminals  of  the  armature  coils,  and  therefore 
the  measured  line  voltage.  In  this  way,  if  the  line  voltage 
is  found  to  be  1,732  volts,  the  voltage  of  any  conductor 

with  respect  to  the  common  center  is  -?-=-  =  1,000. 

V3 

The  Y  E.M.F.  of  a  standard  three-phase  unitooth  ma- 
chine* under  full  load  is  shown  in  Fig.  31. 

The  "delta"  E.M.F.,  or  curve  of  pressure  between  any 
two  of  the  line  wires  under  the  above  condition  of  load,  is 
shown  in  Fig.  32.  This  last  curve  can  be  readily  obtained 
by  uniting  the  Y  curves  for  any  particular  condition  of 
load,  displaced  60°. 


38  POLYPHASE    APPARATUS    AND    SYSTEMS. 


CHAPTER    III. 
GENERATORS   (CONCLUDED). 

Field  Excitation  and  Compounding.  —  The  voltage  of  a 
generator  may  be  maintained  uniform,  under  all  normal 
conditions  of  load,  by  varying  the  strength  of  the  field 
excitation.  For  local  lighting  and  power  distribution 
where  the  circuits  have  fairly  equal  loads,  an  automatic 
or  compounding  arrangement,  as  it  is  called,  is  generally 
desirable.  The  same  results  are  obtained,  in  a  measure, 
by  using  generators  of  good  regulation  and  proper  fre- 
quency, and  sufficient  line-copper  to  -keep  the  loss  down 
to  within  a  very  few  per  cent.  Generators  of  greater 
capacity  than  300  K.W.  are  not,  as  a  rule,  automatically 
compounded,  on  account  of  the  difficulty  of  commutating 
heavy  currents.  Generators  for  long-distance  transmission 
work  are  also  without  this  device  ;  for,  besides  being  usually 
of  large  capacity,  they  are  required  to  take  care  of  heavy 
voltage  drops  in  the  transmission  apparatus,  and  of  pres- 
sure variations  due  to  gradually  changing  loads.  These 
voltage  changes  can  best  be  overcome  by  hand  regulation 
of  the  field  excitation. 

The  usual  method  for  producing  automatic  compound- 
ing by  variation  of  the  field  excitation  requires  two  sets 
of  field  windings,  —  a  shunt  winding  for  the  current  from 
an  outside  source,  and  a  series  winding  for  the  current  ob- 
tained from  the  commutation  of  the  alternating  currents 


GENERATORS. 


39 


of  the  various  phases.  Fig.  33  shows  the  connections  of 
a  General  Electric  three-phase  generator  with  composite 
field  windings.  A  three-part  commutator  rectifies  the  cur- 
rents from  each  of  the  three-phase  circuits,  so  that  un- 
balancing in  any  one  line  has  a  minimum  effect  on  the 
regulation.  The  rotating  shunt  is  practically  the  common 
center  of  the  coil,  giving  a  current  in  the  -series  field,  due 
to  about  1  per  cent 
of  the  terminal  vol- 
tage. The  stationary 
shunt  is  adjustable, 
and  can  be  varied  for 
loads  of  different 
power  factors.  It  also 
serves  to  prevent 
sparking  at  the  com- 
mutator. The  con- 
nections of  the  mono- 
cyclic  generator  are 
similar,  except  that 
the  commutator  recti- 
fies the  current  of  the 
main  circuit  only.  In 
the  Westinghouse 
polyphase  generator 

(Fig.  34),  the  low  potential  current  for  the  serie$  field  is 
derived  from  a  series  transformer  within  the  armature. 
All  phases  are  represented  in  the  primary.  The  com- 
pounding field  current  depends  upon  the  sum  of  the  cur- 
rents flowing  in  the  circuits  supplied  by  the  armature. 

The  demagnetizing  effect,  and  consequent  reduction  of 
voltage,  due  to  a  load  of  poor  power  factor,  has  been  ex- 


Conn  nr->\_it,a>t.oi- 


Fig.  33. 


4o 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


plained.  A  generator  so  loaded  requires  a  greater  field 
excitation  than  when  running  on  non-inductive  load.  The 
comparative  voltages  with  loads  of  varying  power-factor, 
and  the  same  excitation  is  shown  in  Fig.  35.  Curve 
a  is  the  compounding  when  lights  are  the  chief  load, 
and  b  the  curve  when  the  load  consists  chiefly  of  motors. 


Auxiliary  Field, 

rMlOQQOQOOQO 


Fig.  34. 

It  will  be  seen  that  a  generator  properly  over-com- 
pounded for  a  night  load  of  lamps  will  not  give  the 
proper  voltage  for  a  day  load  of  motors.  The  stationary 
shunt  in  Fig.  33  will  then  have  to  be  adjusted  for  the  vary- 
ing character  of  the  load.  The  monocyclic  generator  is 
an  exception,  in  that  this  adjustment  is  not  necessary,  as 


GENERATORS.  41 

.will  be  shown  later.  Automatic  compounding  may  also  be 
attained  by  variation  of  the  current  in  the  shunt  windings 
alone.  In  this  method  the  exciter  E.M.F.  is  varied  by  an 
arrangement  of  solenoids  and  magnetic  plungers,  acting  on 
the  exciter  rheostat. 

The  energy  required  to  excite  the  fields  of  good  com- 
mercial generators  on  a  non-inductive  load  varies  from 
about  i  per  cent,  in  the  case  of  generators  of  500  K.W. 
capacity  and  over,  to  2,  and  sometimes  3,  per  cent  in 


Kilowatts  Output 
Fig.  35. 

smaller  machines.  The  dynamo  supplying  the  separate 
exciting  current  must,  of  course,  be  of  greater  capacity 
when  the  alternating  generator  is  non-compounded,  and 
does  not  furnish  a  portion  of  the  exciting  current. 

The  exciting  dynamos  are  usually  driven  from  a  pulley 
on  the  shaft  of  the  alternating  generator.  In  large  water- 
power  plants  the  best  practice  is  to  drive  the  exciters 
from  separate  water-wheels,  and  in  steam  plants  from  sepa- 
rate engines.  By  this  method  any  variation  in  the  gene- 
rator speed  is  without  effect  on  the  exciting  current. 


42     POLYPHASE  APPARATUS  AND  SYSTEMS 

Regulation.  —  Inherent  regulation  is  defined  in  four  or 
five  different  ways  ;  but  the  now  commonly  accepted  defi- 
nition is  the  percentage  rise  of  the  voltage  when  full  non- 
inductive  load  is  thrown  off,  the  generator  speed  and  the 
field  excitation  remaining  constant.  From  what  has  been 
said  in  connection  with  armature  windings,  it  follows  that,  as 
a  rule,  generators  with  unitooth  armatures  will  not  have  as 
good  a  regulation  as  the  multitooth  type.  However,  good 
regulation  in  these  machines  can  be  obtained  at  a  slight 
sacrifice  of  efficiency,  or  by  using  more  copper  in  the  con- 
struction of  the  generator,  and  thus  increasing  its  cost, 
or  by  the  use  of  a  high  magnetic  saturation  of  the  iron. 
A  certain  three-phase  unitooth  machine  of  large  output 
gave  a  regulation  of  6£  per  cent,  from  full  load  to  10  per 
cent  of  the  load.  The  same  generator,  when  the  load  in 
one  circuit  was  reduced  50  per  cent,  did  not  rise  in  voltage 
more  than  5^  per  cent ;  and  with  no  load  on  one  of  the 
circuits,  the  others  being  fully,  loaded,  the  greatest  varia- 
tion was  8  per  cent.  The  standard  belt-driven  machines 
of  the  unitooth  construction  regulate  within  10  per  cent, 
which  is  close  enough  for  satisfactory  results  to  be  ob- 
tained, even  without  automatic  compounding.  Generators 
of  the  multitooth  construction  require  less  compounding. 
The  standard  machines  of  this  type  have  a  regulation  of  6 
per  cent  or  less. 

On  inductive  loads  the  regulation,  of  course,  is  not  so 
good.  The  generator  mentioned  above  as  having  a  non- 
inductive  regulation  of  6|  per  cent,  will  require  nearly 
1,200  more  ampere  turns  in  the  field  to  give  full-load  vol- 
tage when  it  is  supplying  current  to  motors,  the  power 
factor  of  the  circuit  being  80  per  cent.  The  regulation 
under  these  conditions  is  about  16  per  cent.  These 


GENERATORS .  43 

results  are  immensely  superior  to  those  obtained  with 
the  old  iron-clad  alternators,  which  often  required  30  to 
50  per  cent  increase  in  exciting  current  on  non-inductive 
loads  to  maintain  constant  pressure. 

The  construction,  resulting  in  poor  regulation,  is  some- 
times used  in  generators  designed  for  special  purposes ; 
for  instance,  in  alternating  arc  lighting  where  a  constant 
current  is  required.  Generators  of  a  high  inherent  reg- 
ulation are  sometimes  used  in  certain  kinds  of  electric 
smelting,  where  a  constant  watt  output  is  required.  The 
process  is  started  at  a  certain  voltage ;  and,  as  the  resist- 
ance decreases,  the  voltage  falls  in  inverse  ratio  to  the 
increase  of  current. 

Efficiency  and  Losses.  —  Fig.  36  gives  the  efficiency 
curves  of  a  750  K.W.  three-phase  generator,  and  shows  the 
individual  losses  in  the  machine.  It  will  be  noted  that 
the  highest  efficiency  is  reached  a  little  above  full  load, 
the  losses  being  only  about  5^  per  cent  of  the  total  out- 
put. The  efficiency  at  half  load,  91  per  cent,  is  most 
excellent,  while  84  per  cent  for  one-fourth  load  is  almost 
as  high  as  can  be  expected.  This  generator  was  designed 
for  direct-coupling  to  a  water-wheel ;  so  the  friction  loss  is 
mainly  due  to  two  bearings,  and  is  constant  at  about  I  per 
cent  for  all  loads.  The  I?R.  loss  in  the  field  varies  little 
from  no  to  full  load,  showing  that  the  generator  is  easy  to 
regulate.  The  copper  loss  in  the  armature  is  the  smallest 
loss.  The  core  loss  varies  from  19  K.W.,  at  no  load,  to  24 
K.W.,  at  full  load.  Generators  for  engine  connection  will 
have  an  apparently  higher  efficiency,  especially  at  light 
loads,  as  the  friction  losses  are,  as  a  rule,  reduced  by  the 
omission  of  all  bearing  losses,  these  being  considered  as 
among  the  engine  losses. 


44     POLYPHASE  APPARATUS  AND  SYSTEMS. 


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GENERATORS.  45 

The  efficiencies  of  generators,  as  usually  given,  do  not 
include  the  losses  in  the  exciter.  As  the  exciter  efficiency 
is  from  80  to  90  per  cent,  and  the  I?R.  loss  in  the  fields 
is  about  i  per  cent,  the  reduction  of  the  generator  effi- 
ciency due  to  this  source  will  seldom  be  greater  than  .2  per 
cent,  almost  outside  the  range  of  commercial  accuracy. 

Speed.  —  As  the  frequency  of  an  alternating-current 
generator,  with  a  given  number  of  revolutions  per  minute, 
determines  its  number  of  poles,  it  follows  that  a  high-fre- 
quency generator,  operating  at  a  normal  speed  of  from,  say, 
300  to  600  R.P.M.,  requires  numerous  poles.  The  old- 
style  alternators  of  moderate  output,  and  of  frequencies  of 
125  to  133  cycles,  ran  at  from  1,500  to  2,000  revolutions, 
and  had  10  to  8  poles.  To  maintain  this  high  frequency,  and 
reduce  the  speed,  thereby  increasing  the  number  of  poles, 
results  in  an  expensive  and  inefficient  machine.  A  much 
better  machine,  having  a  speed  of  300  or  600  revolutions, 
is  obtained  by  reducing  the  poles  to  24,  or  1 2,  giving  a  fre- 
quency of  60  cycles.  The  majority  of  polyphase  belt- 
driven  generators  in  actual  operation  are  wound  for  60 
cycles.  Standard  belt-driven  generators  of  this  frequency 
have  the  following  number  of  poles  and  speeds  for  the  re- 
spective outputs  : 

K.W.        POLES.        R.P.M. 
50  8  900 

75  8          9°° 

ioo  8  900 

100  10  720 

150  12  600 

250  16  450 

500  24  300 


46  POLYPHASE   APPARATUS    AND    SYSTEMS. 

The  alternating-current  generator  is  far  from  being  like 
a  direct-current  machine,  —  a  flexible  piece  of  apparatus 
in  respect  to  speed.  The  speed  cannot  be  altered  more 
than  10  per  cent  either  way  from  that  for  which  it  was 
designed,  without  appreciably  affecting  the  constants  of 
the  generator  and  the  apparatus  to  which  the  generator  is 
supplying  current. 

Parallel  Running.  —  In  modern  alternating-current 
plants  of  large  capacity,  especially  long-distance  power 
transmission  plants,  parallel  operation  is  necessary  in 
order  to  effect  a  reduction  in  the  number  of  circuits  and 
transmission  lines.  Other  advantages  are  economy,  sim- 
plicity, and  reliability  of  operation.  Polyphase  generators, 
as  now  designed,  can  be  operated  in  multiple  without  any 
difficulty. 

The  principal  requirement  in  the  generators  is  that  they 
shall  have  a  moderate  armature  impedance.  Too  small  an 
impedance  permits  an  excessive  exchange  of  current  with 
slight  inequality  of  the  field  excitation  of  the  machine,  and 
a  dangerous  flow  if  the  generators  are  connected  up  when 
they  are  not  quite  in  step.  Generators  having  a  large 
armature  impedance  will  operate  in  parallel ;  but,  owing  to 
the  small  synchronizing  current  that  can  be  exchanged, 
the  condition  is  not  stable,  and  the  generators  are  liable 
to  alternately  lead  in  speed  or  "hunt."  When  a  number 
of  generators  are  to  be  run  in  parallel  the  excitation  of 
each  one  should  be  separately  adjusted  to  give  the  same 
current,  otherwise  there  will  be  an  exchange  of  current. 

The  requirements  of  the  prime  mover  are  uniform  speed 
and  uniform  angular  rotation.  In  belt-driven  generators 
the  pulleys  must  all  have  the  same  dimensions.  The  belts 
must  be  watched  to  see  that  they  do  not  slip.  These  two 


GENERATORS.  47 

points  must  be  especially  observed  in  generators  driven 
from  the  same  shaft.  The  speed  regulation  of  engines 
operating  direct-connected  alternators  in  parallel  is  dis- 
cussed in  the  following  section.  Water-wheels  have  an 
absolutely  uniform  angular  rotation,  and  are  the  best  prime 
movers  for  parallel  running. 

Synchronism  of  two  polyphase  generators  is  determined 
by  some  form  of  phase  indicator.  The  commonest  arrange- 
ment consists  of  two  transformers,  the  primaries  of  which 
are  connected  to  each  generator,  care  being  taken  that  the 
connections  are  made  to  similar  phases.  The  secondaries 
are  connected  in  series  with  one  or  two  lamps  in  circuit. 
The  machines  are  in  synchronism  when  the  lamps  cease 
to  glow.  They  may  then  be  thrown  in  parallel  by  the 
main  switches.  With  composite  field  machines  the  com- 
mutators must  be  connected  by  an  equalizer  to  place  the 
series  windings  in  multiple.  The  connections  and  station 
instruments  required  for  the  process  of  throwing  genera- 
tors in  parallel,  and  operating  them  continuously,  as  used 
extensively  in  this  country,  are  shown  in  Fig.  37. 

It  does  not  follow  that,  because  one  phase  of  a  polyphase 
circuit  is  synchronized,  the  other  phases  are  ready  for 
parallel  connection.  It  is  quite  important  that  when  a 
number  of  machines  are  first  installed  for  operating  in 
parallel,  the  connections  should  correspond  throughout  in 
all  the  machines.  The  circuits  can  be  tested  out,  for 
proper  connection,  by  means  of  two  sets  of  phase  lamps. 

In  the  diagram  (Fig.  38)  temporary  transformers  are 
shown  connected  to  a  different  phase  of  the  circuit  than 
that  in  which  are  the  permanent  lamps.  Connection 
should  first  be  made  with  the  outside  blades,  as  shown 
by  the  dotted  lines,  to  prove  that  the  two  sets  of  lamps 


48 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


will  operate  together.  By  the  separate  connections  of 
the  temporary  transformers,  it  can  be  ascertained  if  the 
machines  are  properly  connected  to  the  synchronizing 


I  II 


switches.     The  connections  are  correct  when  both  sets  of 
lamps  are  simultaneously  dark. 

Speed  Regulation  of  Engines.  —  Steam-engines  intended 
for  direct  connection  to  alternators,    especially  such   as 


GENERATORS. 


49 


supply  current  to  rotary  converters,  or  are  operated  in 
parallel,  should  have  such  a  rotation  that  the  maximum 
deviation  from  the  position  of  absolutely  uniform  speed 
of  which  never  exceeds  i£°  in  phase,  —  that  is,  i£°  when 
counting  two  complete  poles  as  360°. 

This  means,  that,  in  an  engine  direct-connected  to  an 
alternator  of  2  ;/  poles,  the  position  of  the  revolving  part 

ji° 

should  never  differ  more  than    4    in  circumference  from 

n 

the  position  it  would  have  at  absolutely  uniform  rotation. 

To  Bus  Bars. 


Synchronizing 
Lamps. 


ansformers 


i M/WW 

x VVVwl, 


Switchboard^] 


rrzq 

Temporary        WVWW 


(^Transformer 


TPS.T.  Switch, 
p       p       p 


MAAA — ? 


_MMMA 


To  Genera'Cof-. 
Pig.  38. 

Thus,  in  a  40  polar  alternator,  the  maximum  deviation 
from  the  position  of  uniform  rotation  would  be  — ,  or  ^ 

20 

electrical  degrees. 

The  above  expresses  the  regulation  of  the  engine  as  a 
deviation  in  position  from  that  of  absolutely  uniform  rota- 
tion in  degrees  of  total  circumference,  measured,  for  exam- 
ple, on  the  circumference  of  the  fly-wheel  or  revolving 


50     POLYPHASE  APPARATUS  AND  SYSTEMS. 

member  of  the  direct-connected  alternator.  It  is  called 
the  "variation"  of  the  engine  when  expressed  in  degrees 
circumference,  the  "variation"  of  the  alternating  circuit 
when  expressed  in  degrees  of  phase. 

The  regulation  of  the  engine  can  be  expressed  as  a  per- 
centage of  variation  from  that  of  an  absolutely  uniform 
rotative  speed.  A  close  solution  of  the  general  problem 
shows  that  i£°  of  phase  displacement  corresponds  to  a 
speed  variation,  or  "pulsation,"  with  an  alternator  of  two 
n  poles,  as  follows  : 

In  the  case  of  a  single  cylinder  or  tandem  compound 

engine 2'?5  IP 

11 

A  cross  compound 5-5  /o 

n 

A  working  out  of  the  problem  also  shows  that  the 
momentum  of  the  reciprocating  parts  and  the  distribution 
of  load  between  the  various  engine  cylinders  predominate 
so  much  that  no  better  results  are  obtained  from  a  three- 
crank  engine  than  from  a  two-crank. 

From  the  above  formula  it  will  be  seen  that  a  4O-polar 
alternator,  driven  by  a  cross-compound  engine  or  three- 
cylinder  engine,  gives  a  permissible  pulsation  of  .275  per 
cent  (a  little  over  \  of  I  per  cent).  This  is  relatively 
easy  to  secure  in  a  modern  well-designed  engine. 

Methods  of  Driving  Generators.  —  The  mechanical  coup- 
ling of  a  generator  to  the  prime  mover  is  determined 
mainly  by  the  size  of  the  generator  and  the  type  and  speed 
of  the  prime  mover.  Polyphase  units  up  to  200  K.W.  are 
usually  belted,  unless  the  prime  mover  consists  of  a  water- 
wheel  of  high  speed,  or  special  conditions  favor  direct  con- 
nection to  an  engine.  The  mechanical  arrangement  of 


GENERATORS.  51 

the  generator  parts  is  shown  by  Fig.  39.  The  yoke  rests 
on,  and  is  sometimes  an  integral  part  of,  the  bedplate,  which 
t  also  supports  two  bearings.  The  pulley  is  overhung. 

The  method  of  belt-driving  larger  units  is  shown  in 
Fig.  40.  The  bedplate  is  extended,  and  carries  a  third  or 
outboard  bearing  which  partly  relieves  the  inner  bearing  of 
the  belt  strain  and  the  weight  of  the  pulley. 

Generators  designed  for  water-wheel  connection  are 
usually  provided  with  bedplate  shaft  and  two  bearings. 
These  machines  are  self-contained  for  the  more  perfect 
alignment  of  the  bearings.  Fig.  41  illustrates  the  gene- 
ral arrangement  of  generators  of  500  K.W.  capacity  and 
above.  A  half-coupling  is  provided,  which  is  machined  to 
a  close  fit  with  the  other  half  furnished  with  the  water- 
wheel. 

Generators  for  direct  connection  to  engines  are  built 
without  bedplate,  shaft,  or  bearings.  The  yoke  rests  on  a 
thin  iron  soleplate  supported  by  a  suitable  foundation. 
The  engine  bearing  serves  also  for  the  inner  bearing  of 
the  generator.  The  outboard  bearing  rests  on  a  separate 
cap.  It  is  usually  furnished  with  the  engine,  and  is  of  a 
design  uniform  with  the  inner  bearing.  The  engine  shaft 
extended  carries  the  revolving  element  of  the  electrical 
unit  (Fig.  42). 

Polyphase  generators  above  500  K.W.  should  preferably 
be  direct-coupled  to  the  prime  mover.  The  method  of 
driving  large  generators  by  belts  or  ropes  necessitates  a 
large  extension  of  the  base,  and  a  heavy  pulley,  and  is 
mechanically  awkward.  This  method  of  driving  may  be 
used  in  exceptional  cases,  as,  for  instance;  in  connection 
with  a  wheel  plant  already  installed,  operating  under  a 
very  low  head  at  a  low  speed.  The  increased  cost  of 


52  POLYPHASE   APPARATUS    AND    SYSTEMS. 


GENERATORS. 


53 


— '  Cf/7  of  frjfae  //7/?gj  iff  on  for  removing  Speofo. 

~ 


54  POLYPHASE   APPARATUS   AND   SYSTEMS. 

extended  shaft,  outboard  bearing,  and  pulley  will  go  far 
towards  offsetting  the  increased  cost  of  a  slower  speed 
generator,  for  direct  connection,  which  does  not  require 
these  parts. 

Polyphase  generators  are  direct-connected  to  water- 
wheels,  .either  by  a  vertical  or  by  a  horizontal  shaft.  Very 
few  generators  in  this  country  run  from  vertical  turbines. 
The  notable  exceptions  are  the  superb  generators  at  Niag- 
ara Falls,  and  those  in  the  station  of  the  Portland  General 
Electric  Company  at  Oregon  City,  Oregon.  The  advan- 
tages of  the  vertical  connection  lie  in  the  saving  of  floor- 
space,  requiring  a  smaller  power-house,  and  in  more  respon- 
sive wheel  regulation.  The  shafting  is  out  of  sight,  the 
revolving  parts  reduced  to  a  minimum,  and  the  effect,  as  a 
whole,  most  pleasing.  The  disadvantages  are,  the  increased 
cost,  and  a  possible  mechanical  difficulty  in  supporting  the 
vertical  shaft,  weighted  with  the  revolving  electrical  and 
hydraulic  parts.  The  European  practice  is  to  almost  ex- 
clusively employ  the  vertical  generator  in  connection  with 
vertical  water-wheels. 

Horizontal  generators,  for  direct  coupling  to  turbines,  are 
usually  so  constructed  that  the  lower  frame  forms  one  part 
of,  or  rests  on,  a  base,  which  also  supports  the  two  standards. 
Sometimes,  as  shown  in  Fig.  22,  an  extension  and  third 
bearing  is  used,  the  water-wheel,  properly  housed,  taking 
the  place  of  the  pulley.  Such  an  arrangement  is  pecu- 
liarly adapted  for  use  with  impact  wheels.  This  construc- 
tion is  used  in  the  power  plants  of  the  Big  Cottonwood 
Electric  Company,  the  Pioneer  Electric  Company,  Ogden, 
Utah,  and  the  Southern  California  Power  Company,  Red- 
lands,  Cal.  Perfect  and  permanent  alignment  of  bearings 
is  obtained  by  this  construction. 


GENERATORS. 


55 


A  typical  Westinghouse  two-phase  generator  of  the  re- 
type,  for   direct   connection   to  water- 


volving   armature 


Fig.  43. 


wheels,  is  illustrated  in  Fig.  43.  This  generator  is  one  of 
four  in  service  in  the  plant  of  the  Helena  Water  and 
Electric  Power  Company  of  Helena,  Montana.  The  ma- 


56          POLYPHASE  APPARATUS   AND    SYSTEMS. 

chines  are  of  656  K.W.   output,  run  at  150  R.P.M.,  and 
generate  60  cycle  current  at   500  volts.      Eight  325  K.W. 
step-up  transformers  raise  the  voltage  from  500  to  10,000 
volts  for  transmission  over  a  distance  of  eleven  miles. 
Where  engines  are  direct-connected  to  polyphase  gen- 


Fig-.  44. 

erators,  it  is  customary  for  the  electrical  manufacturers  to 
furnish  the  machine  without  shaft,  base,  or  bearings.  For 
the  same  speed,  therefore,  engine-driven  generators  are 
cheaper  than  those  driven  by  water-wheels.  It  must  not  be 
forgotten,  however,  that  engine  speeds  are  limited  by  a 


GENERATORS.  57 

number  of  conditions,  while  water-wheels  are  practically 
limited  in  speed  only  by  the  head  obtainable. 

Fig.  44  illustrates  a  three-phase  generator  of  1,200  H.P. 
capacity,  direct-connected  to  an  engine  running  at  94 
R.P.M. 

This  generator  is  direct-coupled  to  a  Corliss  type  of 
engine  of  1,300  indicated  H.P.,  running  at  94  revolutions. 
It  has  32  poles,  and  gives  a  current  at  5,000  volts  and  a 
frequency  of  25  cycles.  The  armature  windings  consist  of 
96  coils,  three  for  each  pole,  or  two  slots  per  phase  per 
pole.  The  windings  are  Y  connected.  The  field  coils  are 
flat  strip-copper,  I  in.  by  ^  in.,  wound  on  edge,  and  insu- 
lated by  intervening  layers  of  paper.  As  the  exciting  current 
has  a  pressure  of  not  greater  than  120  volts,  the  potential 
at  the  terminals  of  each  field  spool  is  about  four  volts.  The 
efficiency  of  the  generator  is  95^-  per  cent  at  full  load,  94^ 
per  cent  at  three-quarter  load,  92^  per  cent  at  half  load, 
and  87  per  cent  at  quarter-load.  The  regulation  on  non- 
inductive  load  is  6  per  cent,  and  the  exciting  current  about 
1 20  amperes. 

Conditions  Affecting  Cost From  what  has  preceded,  it 

will  be  easily  understood  that  the  first  factor  in  determin- 
ing the  cost  of  a  polyphase  generator  of  a  given  capacity 
and  conditions  of  operation,  is  its  speed.  The  efficiency 
is  another  factor,  and  likewise  the  regulation.  A  genera- 
tor of  high  efficiency  can  be  built  at  a  reasonable  cost,  but 
at  the  expense  of  some  regulation.  The  same  generator 
may  have  good  regulation  at  the  sacrifice  of  efficiency,  and 
cost  no  more.  To  obtain  both  these  constants,  in  an  emi- 
nent degree,  requires  a  liberal  use  of  copper  and  iron,  and 
results  in  an  expensive  machine. 

The  frequency  of  the  current  for  which  a  generator  is 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


designed  is  another  determining  factor  of  the  cost.  With 
a  given  speed,  changing  the  frequency  alters  the  number 
of  poles  ;  correspondingly,  a  reduction  in  the  number  of 
poles  cheapens  a  generator.  Less  exciting  copper  is 


400 


800 


200 


100 


10       20      30      40       50       60 

Per  Cent  Reduction  in  Cost 
Fig.  45. 

needed  ;  for,  while  the  polar  cross-section  is  unchanged, 
the  average  length  of  turn  is  less.  The  number  of  opera- 
tions in  manufacture  and  handling  are  also  considerably 
reduced.  The  effect  of  change  of  frequency  on  cost  is 
most  noticeable  in  very  slow-speed  direct-connected  units. 


GENERATORS.  59 

Take  the  case  of  a  133  cycle  generator,  direct-connected 
to  an  engine,  running  at  approximately  300  revolutions  per 
minute.  To  ^ive  the  proper  frequency,  it  must  have  52 
poles.  By  reducing  the  frequency  to  40  cycles,  1 6  poles 
are  needed  It  must  not  be  forgotten,  however,  that  low- 
ering the  frequency  of  ally  piece  of  alternating  apparatus 
necessitates  an  increase  in  the  iron  of  the  magnetic  cir- 
cuit. Iron,  however,  is  cheap  as  compared  with  copper 
and  price  of  labor.  Of  course,  the  proportionate  saving  is 
not  so  noticeable  in  high  speeds,  nor  when  the  generators 
are  belt-driven,  or  provided  with  parts  that  remain  the  same 
irrespective  of  the  frequency. 

Fig.  45  shows,  in  an  approximate  degree,  the  relative 
reduction  in  cost  with  increasing  speed.  In  using  this 
curve  for  comparison  of  costs,  it  must  be  kept  in  mind  that 
it  is  approximately  correct  only,  and  applies  to  generators 
of  the  same  type,  frequency,  general  constants,  and  condi- 
tions of  operation. 


UNIVERSITY 
CALIFOB 


60          POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER   IV. 
INDUCTION    MOTORS. 

Principles  of  Operation The  induction  motor  can  be 

compared  to  a  direct-current  shunt  motor,  the  essential 
difference  being  that  the  armature  or  working  current  of 
the  shunt  motor  is  led  into  it  by  brushes,  while  the  work- 
ing current  of  the  induction  motor  is  an  induced  or  trans- 
former current.  The  windings  of  the  induction  motor, 
connected  to  the  supplying  circuit,  besides  carrying  the 
exciting  current,  have  the  additional  function  of  supply- 
ing the  transformer  current.  The  principles  of  operation 
of  the  induction  motor  are  thus  seen  to  combine  both 
those  of  a  motor  and  of  a  transformer.  Rotation  may 
be  considered  as  being  produced  by  the  revolving  mem- 
ber following  a  shifting  magnetic  field  which  is  the  re- 
sultant of  two  or  more  alternating  fields  differing  in 
phase.  The  explanation  of  the  working  of  the  induction 
motor  by  reference  to  the  rotating  magnetic  field  alone, 
however,  is  apt  to  mislead  and  to  hide  its  true  functions. 

The  two  elements  of  an  induction  motor  are  preferably 
designated  as  primary  and  secondary,  and  sometimes  as 
field  and  armature.  Either  may  be  indifferently  the  rotor 
or  stator. 

When  running  without  load,  the  rotor  speed  is  very 
closely  that  of  the  rotating  field,  and  there  is  a  very  small 
current  induced  in  the  secondary  member.  The  magnetic 


INDUCTION   MOTORS.  6l 

pull  of  this  current  on  the  field  produces  a  feeble  torque. 
The  current,  taken  by  the  primary  member,  or  field,  is 
then  composed  of  the  magnetizing  current  and  that  re- 
quired for  overcoming  magnetic  and  mechanical  friction. 
As  the  power  factor  is  low  at  light  loads,  being  not  more 
than  15  or  20  per  cent  in  most  commercial  motors,  the 
energy  supplied  is  not  much  greater  than  that  consumed 
by  a  shunt  motor  of  the  same"  capacity. 

When  running  under  load,  the  speed  of  the  revolving 
element  falls  away  from  that  of  synchronism,  and  the 
E.M.F.  and  working  current  induced  by  the  relative 
cutting  of  the  lines  of  force,  increase  with  the  difference 
in  speeds.  The  pull  of  this  increased  current  on  the  field 
produces  a  powerful  torque.  The  variation  from  the  speed 
of  synchronism  is  called  the  "  slip,"  and,  within  certain 
limits,  is  proportional  to  the  total  secondary  resistance. 

To  insure  -high  efficiency  and  good  regulation,  the  resist- 
ance of  the  shunt  motor  armature  must  be  kept  as  low  as 
practicable.  For  the  same  reason,  the  windings  of  the 
secondary  of  the  induction  motor  should  have  a  low  re- 
sistance. 

Methods  of  Starting  Motors On  connecting  an  induc- 
tion motor  to  its  supplying  circuit,  there  is  an  excessive 
rush  of  current,  which  can  be  prevented  only  by  the  use  of 
some  device  external  to  the  motor  windings  proper.  There 
are  a  number  of  such  arrangements  for  reducing  the  start- 
ing current  of  motors.  Two  of  these  are  extensively  used 
in  this  country,  and  will  be  described.  The  others  are  of 
less  commercial  importance. 

The  first,  and  probably  most  common  device,  consists 
essentially  of  a  variable  resistance,  which  can  be  cut  in 
or  out  of  circuit  with  the  secondary  winding.  When  the 


62 


POLYPHASE  APPARATUS   AND    SYSTEMS. 


secondary  element  is  the  rotor  or  armature,  this  resistance 
often  occupies  a  space  within  the  armature  spider.  It 
may  be  of  copper  strips,  or  —  as  is  usually  the  case  —  of 
iron  cast  into  a  compact  grid  form,  having  a  number  of 
contact  points.  The  whole  of  this  resistance  is  in  series 
with  the  secondary  winding  at  starting.  As  the  motor 


Fig-.  46. 

attains  speed,  a  circular  short-circuiting  switcn,  mounted 
in  a  ring  encircling  the  shaft,  is  pushed  centrally  by  a 
lever,  thus  cutting  out  the  resistance  in  as  many  succes- 
sive steps  as  there  are  contact  points.  Motors  provided 
with  this  starting  device  are  usually  designed  to  start  with 
a  torque  ranging  from  75  to  150  per  cent  of  full-load 
torque.  This  motor  has  the  desirable  characteristic  that 


INDUCTION   MOTORS. 


the  current  is  very  nearly  proportional  to  the  torque  from 
starting  to  full-load  speed.  Fig.  46  illustrates  a  motor  of' 
this  construction,  made  by  the  General  Electric  Company. 
In  some  motors  of  European  make,  an  external  rheostat 
is  used  to  cut  down  the  induced  current.  When  the  sec- 
ondary revolves,  collector  rings  are  required  to  convey  the 
induced  current  to  the  rheostat.  When  the  primary  is 
the  revolving  element,  collector  rings  are  also  needed  to 
supply  the  main  current  to  the  motor. 


Phase  A 


•O- 


Phase  B 


O- 

i 


Fig.  47. 

A  water  rheostat  is  sometimes  employed  abroad,  -by 
means  of  which  the  induced  current  is  varied,  its  strength 
varying  with  the  depth  to  which  the  plates  are  immersed. 
With  this  device,  the  current  taken  by  the  motor  is  closely 
proportional  to  the  torque,  from  starting  to  full-load  speed. 

The  second  method  of  starting  induction  motors  con- 
sists in  reducing  the  impressed  volts  by  the  use  of  some 
form  of  reactance  or  of  compensator  coils,  or  of  resistance 
in  the  main  circuit.  A  compensator  is  the  most  efficient 
means  of  cutting  down  the  voltage,  and  the  most  gen- 


64 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


erally  employed,  one  coil  being  required  for  each  phase. 
The  connections  of  a  Westinghouse  two-phase  motor  and 
starting  device  are  shown  in  Fig.  47.  The  starter  consists 
of  two  coils,  sometimes  called  auto-converters  or  com- 
pensators, one  in  each  phase.  Each  coil  is  arranged  to 
give  a  number  of  different  starting-voltages  to  suit  differ- 
ent conditions  of  operation.  Fig.  48  shows  the  connec- 
tions of  this  starting  device  in  detail.  The  switch  is 
down  for  starting  the  motor,  and  after  speed  has  been 
reached,  is  thrown  up  to  its  running  position,  thereby  cut- 


l  Running  Position 


Off  Position 


I  Starting  Position 


Motor 


Fig.  48. 


ting  out  the  compensator.  By  this  arrangement  the  motor 
can  be  started  at  a  distance.  Connections  can  be  made 
from  i  to  either  2,  3,  or  4,  giving  three  different  starting 
electro-motive  forces  and  starting  torques.  The  maximum 
E.M.F.  and  torque  are  obtained  by  connecting  i  and  4  ; 
for  minimum  E.M.F. ,  i  and  2  are  connected.  Fig.  49 
illustrates  a  completed  Westinghouse  two-phase  starting 
device.  The  connections  of  a  starting  compensator  for  a 
three-phase  motor,  as  made  by  the  General  Electric  Com- 
pany, is  shown  in  Fig.  50.  As  in  the  two-phase  starter, 
there  is  a  coil  in  each  phase,  with  a  number  of  taps.  These 


INDUCTION    MOTORS. 


Fig.  49. 


66          POLYPHASE  APPARATUS   AND    SYSTEMS. 


INDUCTION   MOTORS.    .  67 

compensator  starters,  for  use  with  motors  of  15  H.P.  and 
under,  have  three  taps  with  voltages  40  per  cent,  60  per 
cent,  and  80  per  cent  of  running  full-load  voltage.  Com- 
pensators for  motors  above  15  H.P.  have  four  taps,  giving 
voltages  40  per  cent,  58  per  cent,  70  per  cent,  and  85  per 
cent  of  running  full-load  voltage. 

Induction  motors  which  are  put  in  operation  by  the 
first  method,  may  be  designated  as  the  variable  resistance- 
in-armature  type.  They  frequently  have  a  higher  self- 
induction,  and  require  more  copper  and  less  iron.  The 
secondary  winding  is  definite  and  polar.  Consequently, 
motors  of  this  type  are  rather  expensive  to  construct. 
Motors  which  are  used  with  the  compensator  starter  may 
be  designated  as  the  compensator  or  short-circuited-arma- 
ture  type.  They  are  proportioned  so  that  the  primary  and 
secondary  have  a  low  self-induction.  They  contain  a  min- 
imum amount  of  copper  and  a  considerable  amount  of  iron 
in  the  magnetic  circuit,  and  a  short  air-gap.  Their  distinc- 
tive feature  is  the  short-circuited  armature,  which  is  usually 
of  the  squirrel-cage  construction.  They  are,  therefore, 
cheaper  motors  to  build. 

In  starting  an  induction  motor  with  variable  secondary 
resistance,  precaution  must  be  taken  that  the  resistance 
is  all  in,  otherwise  the  flow  of  current  may  overheat  the 
motor  or  overload  the  lines.  The  armature  lever  should 
be  pulled  out  as  far  as  it  will  go ;  then  the  line  switch 
may  be  closed,  and,  finally,  the  short-circuiting  switch  may 
be  slowly  closed.  The  motor  should  be  handled  at  starting 
to  reach  full  speed  in  about  fifteen  seconds.  As  the  sec- 
ondary resistance  is  of  a  capacity  only  to  start  the  motor, 
it  never  should  be  left  in  circuit  or  used  to  regulate  the 
speed  of  the  motor.  The  motor  is  shut  down  by  reversing 
the  operations  of  starting. 


68          POLYPHASE  APPARATUS   AND   SYSTEMS. 

As  the  drop  in  a  good  transformer  on  a  lightning  load  is 
within  3  per  cent,  and  on  an  inductive  load,  as  motors, 
seldom  less  than  5  per  cent,  it  is  advisable  to  always  use 
separate  transformers  for  lights  and  for  motors.  The 
exception  to  this  rule  is  in  a  secondary  system  of  distribu- 
tion, where  the  motor  load  is  a  proportionately  small  part 
of  the  entire  load. 

Induction  motors  are  sometimes  started  by  being  con- 
nected directly  to  the  supplying  circuit  without  the  use  of 
any  form  of  starting  device.  Such  a  motor  will,  of  course, 
take  a  large  starting  current.  This  can  be  kept  down  by 
making  the  resistance  of  the  armature  conductors  rather 
high,  and  by  confining  the  motor  to  work  requiring  a  small 
starting  torque.  A  motor  started  in  this  way  should  not 
be  used  on  circuits  where  the  effect  of  a  large  starting 
current  on  the  potential  regulation  of  the  system  is  of  im- 
portance. 

A  three-phase  induction  motor  is  reversed  by  changing 
any  two  of  the  leads,  and  a  two-phase  by  changing  the  two 
leads  of  either  phase. 

Construction  of  Primary  and  Secondary The  simple 

and  substantial  construction  of  the  induction  motor  is  one 
of  its  chief  advantages,  resulting  in  a  minimum  cost  of 
maintenance  and  attendance,  and  offsetting  its  compara- 
tively high'  first  cost.  While  either  element  may  be  the 
rotor,  by  far  the  larger  number  of  commercial  motors  are 
now  constructed  with  a  fixed  primary  and  with  a  rotor 
secondary. 

The  fixed  primary  may  be  likened  to  an  inverted  arma- 
ture. It  is  built  up  of  slotted  laminations  mounted  on  a 
cast-iron  spider.  The  coils  are  imbedded  in  the  slots.  Fig. 
5 1  illustrates  a  Westinghouse  primary  or  field  ready  to  re- 


INDUCTION    MOTORS. 


69 


Fig.  51. 

ceive  its  conductors.  These  stationary  windings  are  usu- 
ally protected  from  mechanical  injury  by  end  shields,  which 
frequently  support  the  bearings.  The  Westinghouse  Com- 


/O          POLYPHASE   APPARATUS   AND    SYSTEMS. 

pany  employ  this  form  of  construction  in  even  the  largest 
sizes,  as  illustrated  in  Fig.  52,  which  represents  a  500  H.P. 
motor. 

This  motor  is  wound  for  three-phase  current  at  60  cycles 


Fig.  52. 


and  400  volts.  It  has  36  poles,  running,  therefore,  at  200 
R.P.M.  The  secondary  has  a  squirrel-cage  winding,  bar 
wound  as  is  the  primary.  The  starting  torque  is  two  and 
one-fourth  times  the  full-load  rated  torque.  The  drop  in 


INDUCTION   MOTORS.  ?I 

speed  from  no  to  full  load  is  4  per  cent.  The  power  factor 
at  full  load  is  given  as  93  per  cent.  The  dimensions  are : 
Height,  10  feet  3  inches ;  floor  space  occupied,  9  feet  6 
inches  by  3  feet  6  inches  ;  diameter  at  air  gap,  7  feet. 
The  total  weight  is  42,000  pounds.  This  motor  is  direct- 
coupled  to  a  line  shaft,  driving  a  mill  in  Mexico. 

The  rotor  armature  of  the  standard  form  of  motor  has 
a  laminated  slotted  structure  similar  to  the  primary.  In 
motors  of  the  variable  resistance  type,  the  secondary  has  a 


Fig.  53. 

definite  series  of  coil  windings,  corresponding  to  the  polar 
windings  of  the  primary.  Motors  of  the  short-circuited 
type  are  generally  wound  with  copper  bars  laid  in  the  slots 
and  connected  at  both  ends  by  short-circuiting  metal  rings. 
Secondaries  of  this  construction  are  termed  squirrel-cage 
armatures.  Fig.  53  shows  an  armature  wound  in  the 
manner  described  and  illustrative  of  this  type. 

In  the  Stanley  Company's  motor  (Fig.  54)  the  field  is 
stationary.  There  are  in  reality,  two  fields  and  two  arma- 
tures. The  secondary  windings  are  connected  so  that  the 


72    POLYPHASE  APPARATUS  AND  SYSTEMS. 

wire  lying  under  the  field  poles  on  one  armature  is  in  series 
with  the  wire  lying  between  the  poles  on  the  other.  The 
field  coils  are  staggered,  each  half  alternately  playing  the 
part  of  a  motor  and  transformer. 

Starting  Torque  and  Current.  —  At  normal  voltage  cer- 
tain types  of  motors  possessing  a  moderate  secondary  resist- 


Fig.  54. 

ance,  —  as,  for  instance,  a  motor  of  the  variable  resistance 
type,  with  the  resistance  cut  out,  —  will  have  a  small  start- 
ing torque  due  to  the  reaction  of  the  excessive  induced 
secondary  current,  on  the  primary.  The  starting  current 
consumed  by  the  motor  will  likewise  be  excessive.  At 
nearly  synchronous  speed  such  a  motor  will  have  a  pow- 


INDUCTION   MOTORS. 


73 


erful  torque.  By  increasing  the  secondary  resistance,  the 
starting  torque  is  raised  until  a  critical  resistance  is  reached, 
beyond  which  point  the  starting  torque  decreases. 

The  starting  torque  of  an  induction  motor  is  also  de- 
pendent upon  the  potential  applied  at  its  terminals.  The 
starting  current  is  reduced  by  lowering  the  voltage,  but  at 


IS* 


I 


Standstill 


Armature  Slip 
Fig.  55. 


Synchronism 


the  sacrifice  of  the  torque  at  starting,  which  varies  as  the 
square  of  the  volts. 

An  inspection  of  the  curves  in  Fig.  55  will  show  how 
the  starting  torque  is  influenced  by  varying  the  secondary 
resistance.  The  secondary  winding  of  the  motor  is 
assumed  to  have  a  fixed  resistance  of  .02  ohms.  At  start- 
ing, a  variable  resistance  is  connected  in  series,  making  a 
total  of  .18  ohms.  The  corresponding  torque  is  about  25 


74         POLYPHASE  APPARATUS   AND   SYSTEMS. 

pounds,  or  150  per  cent  of  full-load  torque.  When  the 
motor  reaches  about  50  per  cent  of  synchronism,  part  of 
the  resistance  is  cut  out,  making  the  total  .045  ohms.  The 
torque  now  increases  until  about  85  per  cent  of  synchro- 
nous speed  is  reached,  when  it  begins  to  drop.  At  this 
point  the  remaining  resistance  is  short-circuited,  leaving 
only  the  resistance  of  the  secondary.  The  torque,  due  to 
this  resistance,  .02  ohms,  reaches  its  maximum  at  about 


«r  100 


-500--^ 


20   40 


GO   80  100  120  140  100  "180  200  220  240  260  2dO  300  320  340 

Horse  Power  Output 
Fig.  56. 


90  per  cent  of  synchronism.  The  starting  torque,  with  a 
secondary  resistance  of  .02  ohms,  is  about  seven  pounds. 
The  starting  torque,  due  to  a  resistance  of  .75  ohms,  is  less 
than  when  the  total  secondary  resistance  is  .18  ohms,  be- 
ing only  1 6  pounds.  The  current  in  the  primary  of  such 
a  motor  at  all  speeds  will  be  nearly  proportional  to  the 
torque  developed.  At  the  moment  of  cutting  out  the  suc- 
cessive resistances,  the  current  will  momentarily  increase 
in  strength.  It  can  be  readily  seen  that,  by  using  a  suf- 


INDUCTION   MOTORS.  75 

ficient  number  of  resistance  steps,  the  motor  could  be 
brought  up  to  speed  with  uniform  torque  and  current. 
When  the  motor  is  taxed  beyond  its  capacity,  its  torque 
and  speed  rapidly  diminish  and  a  large  current  will  flow. 
This  break-down  point  is  determined  by  the  design  of  the 
motor,  and  is  fixed  at  from  50  to  100  per  cent  greater 
than  the  rated  load.  The  working  point  of  such  a  motor 
is  on  the  descending  portion  of  the  power  curve,  at  about 
two-thirds  of  the  maximum  output.  Curves  of  torque  and 
amperes  output  at  all  loads,  of  a  175  H.P.  motor,  are  given 
in  Fig.  56. 

The  magnetizing  current  which  is  characteristic  of  most 
alternating-current  apparatus,  such  as  transformers,  induc- 
tion motors,  etc.,  has  the  effect  of  increasing  the  full-load 
current  and  putting  a  greater  demand  on  transformers, 
line,  and  generators.  The  total  current  is  greater  than 
that  actually  required  in  supplying  the  losses  and  doing 
the  work  of  the  motor.  The  ratio  of  this  working  or 
energy  current  to  the  total  current  gives  the  power  factor. 

As  the  starting  current  of  the  motor,  with  short-circuited 
armature,  is  reduced  by  lowering  the  voltage,  it  follows 
that,  for  the  same  starting  torque  as  that  developed  by  the 
variable  resistance  type,  the  current  will  be  considerably 
greater. 

The  line  starting  current  and  the  torque  of  some  makes 
of  motors  with  short-circuited  armatures,  expressed  in  per- 
centages of  full  load,  are  about  as  follows : 

E.M.F.  STARTING  CURRENT.  STARTING  TORQUE. 
40%                                          112%  32% 

60$  250$  72% 

80$  450$  128$ 

100$  700$  200$ 


76    POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  local  current  between  the  compensator  and  motor 
will  be  greater  than  the  line  starting  current,  as  its  poten- 
tial is  lower.  The  action  of  the  compensator  is  similar  to 
that  of  a  transformer. 

By  increasing  the  resistance  of  the  armature  of  these 
motors,  the  starting  current  for  the  same  torque  is  de- 
creased ;  but  the  result  is  a  loss  of  efficiency,  which  may 
be  as  great  as  2  or  3  per  cent. 

The  following  is  a  comparison  of  the  values  of  torque 
and  starting  current  of  two  10  H.P.,  220  volt,  60  cycle 
motors,  of  the  high  inductance  and  variable  resistance,  and 
of  the  short-circuited  types  : 

VARIABLE  RESIST-    SHORT-CIRCUITED 
ANCE  ARMATURE.  ARMATURE. 

Running  current,  full  load      .     .  27.6  amperes    26.7  amperes 

"  "        no      "         .     .  8.3         "  12.6        " 

Starting      current,      maximum 

torque 60  "         174  " 

Starting  current,  full-load  torque  28  "         100  " 

"  f  "  "  20  "  67  " 

Torque,  full  load 45  pounds         45  pounds 

"         maximum,  starting    .     .  79        "  79        " 

Horse-power,  maximum      ...  16  H.P.  28  H.P. 

Drop  in  speed,  full  load     .     .     .  2.2  per  cent      2.5  per  cent 

Rise  in  temperature       .     .     .     .  24°  C.  20°  C. 

Weight  with -starting  device    .     .  1050  pounds  943  pounds 

The  characteristics  of  the  variable  resistance  type  of 
high  inductance,  as  ordinarily  built,  may  be  summed  up  as 
follows  : 

Medium  break-down  point. 
Small  magnetizing  current. 

High  power  factor  and  efficiency  at  all  loads  up  to 
and  including  full  load. 


INDUCTION   MOTORS.  77 

Torque  proportional  to  the  starting  and  running  cur- 
rent. 
Small  percentage  drop  in  speed. 

The  characteristics  of  the  short-circuited  armature  type 
of  low  inductance  are  : 

High  break-down  point. 
Fairly  large  magnetizing  current. 
High   power  factor  and  efficiency  at  full  and  over- 
loads. 

Large  starting-current  for  starting  torque. 
Greater  percentage  drop  in  speed. 

Motors  of  the  first  type  will  be  seen  to  have  a  special 
range  of  usefulness  when  operated  from  circuits  requiring 
good  regulation  such  as  is  demanded  in  central  station 
work.  They  are  desirable  for  service  where  the  motors 
are  apt  to  run  considerably  underloaded. 

The  second  type  is  to  be  recommended  for  power  cir- 
cuits, and  when  the  motors  must  be  started  from  a  dis- 
tance and  simplicity  of  operation  is  of  moment.  It  is 
adapted  for  service,  calling  for  low  starting  efforts  and 
constant  full  load,  and  is  especially  advantageous  when 
the  motors  are  apt  to  run  overloaded,  or  on  circuits  of 
varying  voltage.  It  is  not  adapted  for  lighting  circuits, 
where  good  regulation  is  important,  unless  the  current 
at  starting  is  small  when  compared  with  the  capacity  of 
feeders  and  generators.  Fig.  57  represents  a  75  H.P., 
three-phase  motor  of  this  type,  made  by  the  General 
Electric  Company. 

The  fields  of  both  types  of  motors  may  be  constructed 
to  have  a  low  self -inductance,  in  which  case  both  motors 


78    POLYPHASE  APPARATUS  AND  SYSTEMS. 

will  possess  the  same  general  characteristics,  except  as  to 
the  relation  of  starting  current  to  the  starting  torque. 

Speed  Regulation Absolutely    synchronous    speed   is 

never  attained  in  an  induction  motor,  as  some  slip  is 
required  to  furnish  the  current  consumed  by  the  light- 
load  losses.  Under  increasing  load,  the  speed  will  fall 
away  from  synchronism  until  the  break-down  point  is 


Fig.  57. 

reached,  and  if  the  motor  is  not  relieved  of  its  load,  it  will 
come  to  a  standstill.  The  current  will  then  be  at  its  max- 
imum. The  fall  in  speed  from  that  at  light  load  to  that 
at  normal  rated  load  will  vary  in  some  types  of  induction 
motors  from  i^  per  cent,  as  in  motors  of  100  H.P.,  to 
3  per  cent,  as  in  smaller  motors.  Motors  constructed 
with  high  and  fixed  secondary  resistances  may  drop  in 
speed  as  much  as  9  per  cent. 


INDUCTION   MOTORS. 


79 


The  complaint  has  been  made  against  the  induction 
motor  that  it  is  an  inflexible  piece  of  apparatus  in  respect 
to  regulation  of  speed.  It  is  quite  true  that  wide  varia- 
tions of  speed  are  obtained  in  modern  motors  only  at  the 
expense  of  efficiency  and  increased  cost  of  construction. 

There  are  a  number  of  possible  methods  of  obtaining 
a  variation  of  speed  in  an  induction  motor,  three  of  which 
will  be  described  ;  only  two  of  which,  however,  are  at  the 
present  time  generally  employed. 


Controller 


Fig.  58. 

The  method  now  most  employed  is  that  by  rheostatic 
control.  A  resistance  is  intercalated  in  the  secondary 
circuit,  which  can  be  varied  by  short  successive  steps. 
The  range  of  speed  usually  demanded  of  a  variable  speed 
induction  motor  does  not  permit  the  use  of  the  small  re- 
sistance, such  as  is  used  in  starting  in  some  designs  of 
motor,  and  which  is  located  within  the  rotor  armature. 
An  external  rheostat  is  required,  of  sufficient  size  to  dissi- 
pate a  considerable  amount  of  energy.  Fig.  58  shows  the 
connections  of  a  three-phase  motor  and  of  a  rheostatic  con- 
troller for  variable  speeds.  Collector  rings,  as  shown,  must 


80         POLYPHASE  APPARATUS  AND   SYSTEMS. 

be  added -to  motors  having  revolving  secondaries  for  elec- 
trically connecting  the  windings  and  external  resistance. 

The  main  line  is  shown  as  passing  through  the  control- 
ler. By  this  arrangement  the  circuit  is  closed  simulta- 
neously with  the  commencement  of  the  operation  of  cutting 
out  the  resistance.  In  large  motors  the  controller  is  sepa- 
rate from  the  resistance,  being  connected  to  it  by  cables. 
It  is  in  appearance  similar  to  the  well-known  street-car 
controller,  and,  like  it,  is  reversible. 

The  speed  of  an  induction  motor  can  also  be  controlled 
by  changing  the  impressed  volts  at  the  motor.  This 
method  requires  the  use  of  an  external  reactance  or  a 
compensator,  and  a  motor  possessing  a  high  fixed  armature 
resistance. 

The  controller  and  compensator  are  usually  separate. 
By  a  sufficient  number  of  taps  in  the  latter,  connected  by 
cables  to  the  controller,  a  graduated  variation  of  the  im- 
pressed volts  is  obtained,  and  a  corresponding  variation  in 
speed. 

The  third  method  of  controlling  the  speed  is  by  chan- 
ging the  number  of  poles.  When  a  variety  of  speeds  is 
required,  this  method  is  complicated,  requiring,  in  addition 
to  a  compensator,  an  elaborate  switching  device.  It  is 
objectionable  also,  as  the  motor  can  only  run  at  full,  one- 
half,  and  one-quarter  speed,  and  at  no  intermediate  speeds. 
This  method  has  been  successfully  employed  in  cases  where 
half  speed  and  half  full-load  torque  are  required. 

An  investigation  of  the  relative  efficiencies  and  power 
factors  of  induction  motors  of  10  H.P.  output,  equipped 
with  the  rheostatic  and  with  the  potential,  variable  speed- 
controlling  devices,  gives  the  approximate  results  shown 
in  the  following  table: 


INDUCTION    MOTORS. 


8l 


SPEED.* 

METHOD  OF 
CONTROL. 

EFFICIENCY. 

P.   F. 

AP.  EF. 

Full 

(    Rheostatic 

83 

86 

72 

(   Potential 

83 

86 

72 

Half  

\   Rheostatic 

41.5 

86 

36 

(   Potential 

36 

57 

20.5 

Quarter  . 

(   Rheostatic 
(   Potential 

21 

16 

86 
48 

18 
7-7 

In  practice  it  will  be  found  that,  in  order  to  give  the 
best  all-round  results,  the  motor  for  potential  control  will 
have  a  lower  efficiency  at  full  speed  than  the  motor  built 
for  rheostatic  speed  control. 

The  motor  with  rheostatic  control  shows  the  same  power 
factor  at  all  speeds. 

The  potential  control  gives  a  lower  power  factor  and 
efficiency  at  all  but  full  speed. 

The  motor  controlled  by  change  of  poles  will  be  found 
to  be  the  most  efficient  for  half  and  quarter  speeds,  and 
has  the  highest  power  factor  except  at  quarter  speeds. 

Of  the  commercial  methods  of  obtaining  speed  variation, 
that  by  potential  control  is  inferior  to  the  rheostatic  con- 
trol in  point  of  efficiency.  The  drawback  to  the  rheostatic 
method  is  that  the  motor  requires  collector  rings. 

Frequency.  —  Induction  motors  of  frequencies  of  from 
25  to  60  cycles,  as  constructed  at  the  present  time, 
have  somewhat  better  power  factors  and  efficiencies  than 
higher  frequency  motors.  Motors  of  a  frequency  of  125 
cycles  or  thereabouts  are  seldom  built  in  sizes  above 
20  H.P.  Motors  of  this  frequency  being  somewhat 
difficult  of  construction  on  account  of  the  small  air  gap 


*  The  torque  is  assumed  to  be  constant  at  all  speeds. 


82    POLYPHASE  APPARATUS  AND  SYSTEMS. 

required  and  the  greater  number  of  poles,  are  not  cheaper 
than  25  cycle  or  60  cycle  motors  of  corresponding  sizes, 
as  might  be  expected.  The  reverse  holds  good  with  lower 
frequencies,  60  cycle  motors  costing  less  to  build  than 
motors  of  25  cycles. 

Twenty-five  cycle  motors  have  the  disadvantage  that  on 
account  of  difficulties  in  the  winding  construction,  the 
speeds  are  practically  limited  to  750,  500,  375,  and  300 
R.P.M.  The  bipolar  motor,  running  at  1,500  revolutions, 
is  limited  to  the  smallest  sizes.  The  slow  speeds  of  300 
and  375  revolutions  make  the  motor,  unless  it  be  one  of 
great  capacity,  an  expensive  piece  of  apparatus.  These 
conditions  limit  the  average  practical  speed  of  25  cycle 
motors,  of  sizes  from  5  H.P.  to  75  H.P.,  to  750  revolutions. 

Frequencies  of  35  to  40  cycles  are  more  desirable  for 
the  average  conditions  of  motor  work,  as  they  permit  a 
much  greater  range  of  commercial  speeds. 

The  frequency  of  60  cycles  likewise  permits  the  con- 
struction of  motors  with  a  wide  range  of  speed,  and  which 
are  comparatively  cheap  to  build  throughout  the  entire 
list. 

Voltage Induction  motors  should  not  be  run  at  lower 

voltages  than  that  for  which  they  are  designed,  as  the  out- 
put varies  with  the  square  of  the  voltage.  For  instance, 
if  the  volts  at  the  motor  are  10  per  cent  lower  than  nor- 
mal, a  motor  which  has  a  maximum  output  of  30  per  cent 

(QO)  2 
greater  than  the  full-load  output  will  give  only  —    -  x 

130=  105  per  cent  of  its  rated  output.  The  margin  is 
too  close  for  continuous  work,  as  it  will  not  take  care  of 
any  sudden  fluctuation  of  load  or  unusual  drop  in  the  line. 
The  output  of  the  motor,  on  a  higher  voltage  circuit  than 


INDUCTION   MOTORS.  83 

that  for  which  it  is  designed,  will  be  increased,  and  the 
current  likewise,  especially  at  light  loads.  Within  ordi- 
nary variation  of  voltages,  the  power  factor  and  efficiency 
at  full  load  remain  practically  unchanged. 

In  laying  out  the  wiring  of  a  motor  which  takes  a  heavy 
starting  current,  allowance  should  be  made  for  this  mo- 
mentary current ;  otherwise  the  impressed  volts  may  drop 
below  the  point  where  the  motor  will  start. 

Motors  with  stationary  fields  could  be  wound  for  fairly 
high  voltage,  but  for  the  distributed  form  of  winding  re- 
quired to  keep  down  self-induction,  the  space  necessary 
for  high  insulation  being  occupied  by  the  conductor. 
Standard  American  motors  below  50  H.P.  are  not  wound 
above  550  volts.  It  is  considered  practical  to  wind  larger 
motors  up  to  3,000  volts.  European  makers,  on  the  other 
hand,  build  motors  of  10  H.P.  to  30  H.P.  for  pressures  of 
500  to  2,000  volts,  motors  of  50  H.P.  for  3,000  volts,  and 
those  of  75  H.P.  and  larger  for  5,000  volts. 

Power  Factor  —  Efficiency It  has  been  seen  that  the 

ratio  of  the  energy  current  of  a  motor,  or  the  current 
required  in  supplying  its  losses  and  doing  the  work  to  the 
total  current  consumed,  gives  the  power  factor.  The  pro- 
duct of  the  power  factor  and  the  actual  efficiency  of  an  in- 
duction motor  gives  the  apparent  efficiency.  This  last 
quantity  determines  the  capacity  of  transformers  and  gen- 
erators required  for  supplying  current  to  the  motors.  As 
has  been  seen,  the  influence  of  the  power  factor  extends 
back  in  the  chain  of  transmission  with  greater  effect  on 
the  supplying  circuit,  necessitating,  in  the  case  of  a  poor 
power  factor,  on  account  of  its  inductive  effects,  an  addi- 
tional increase  in  the  capacity  of  the  transmission  lines. 
For  thi§  reason,  it  is  usually  of  importance  that  induction 


84 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


motors  be  designed  to  give  the  highest  possible  power  fac- 
tor. Where  the  generating  power  is  expensive,  it  is  some- 
times of  more  importance  to  use  motors  of  higher  efficiency 
than  those  of  high  power  factor.  Under  all  circumstances, 
however,  it  is  desirable  to  have  the  apparent  efficiency  of 
the  motors  as  high  as  possible. 

The  power  factors  of  standard  commercial  induction 
motors  of  American  manufacture  vary  at  full  load  from 
•75  to  .92,  depending  upon  the  size  and  frequency  of  the 
motor.  The  efficiencies  range  from  .80  to  .92.  The 
apparent  efficiencies  in  motors  above  5  H.P.  output  will 
be  found,  as  a  rule,  not  less  than  .75.  This  means  that 
the  transformer,  supplying  current  to  induction  motors  of 
average  sizes,  must  have  a  capacity  of  i  K.W.  for  every 
horse-power  output  of  the  motors. 

The  following  table  gives  approximate  capacities  of 
standard  transformers  that  should  be  used  with  two-phase 
and  three-phase  induction  motors  : 


H.  P. 
CAPACITY  MOTOR. 

THREE-PHASE. 

TWO-PHASE. 
2  TRANSFORMERS. 

2  TRANSFORMERS. 

3  TRANSFORMERS. 

I 

.6  K.W. 

.5  K.W. 

.6  K.W. 

2 

i.       " 

.1      " 

i.       " 

3 

2.          " 

1.5      " 

i.S     " 

5 

3-       " 

2. 

3-       " 

1% 

4-        " 

2-5 

4-        " 

10 

5-       " 

3-5      " 

5-        " 

IS 

7-5     " 

5-        " 

7-5      " 

20 

10.           " 

7-5      " 

10.           " 

30 

15.       « 

10.            " 

15.        " 

5° 

25.       « 

15.        " 

25.        " 

75 

25. 

35-       " 

100 

30. 

45- 

INDUCTION   MOTORS.  85 

The  efficiency  of  commercial  induction  motors  can  be 
somewhat  increased  by  not  sparing  iron  and  copper,  as 
the  losses  of  an  induction  motor  are  of  the  same  kind 
as  those  of  a  generator,  consisting  of  copper  loss,  hyste- 
resis loss,  and  friction  loss. 

The  power  factor  can  be  bettered  by  reducing  the  air 
gap  and  iron  density,  and  thereby  lowering  the  magnetiz- 
ing or  ~ wattless"  current.  To  do  this,  however,  and 
retain  high  efficiency,  increases  the  cost  of  the  motor,  and 
it  then  becomes  a  question  whether  the  increased  advan- 
tages are  worth  the  extra  expense.  Mechanical  considera- 
tions limit  the  clearance  between  field  and  armature.  Fig. 
56  shows  the  curves  of  efficiency,  power  factor,  and  appa- 
rent efficiency,  as  well  as  torque  and  ampere  output  of  a  175 
H.P.  motor.  At  full  load  the  efficiency  is  91  per  cent,  the 
power  factor  .88,  and  the  apparent  efficiency  80  per  cent. 
The  efficiency  at  half  load  is  as  good  as  that  at  full  load ; 
and  at  one-quarter  load,  the  efficiency  is  still  well  up,  being 
85  per  cent.  The  break-down  point  is  at  over  twice  full 
load.  The  power  factor  is  highest  at  about  260  H.P., 
being  over  91  per  cent. 

In  many  cases  it  is  desirable  to  design  motors  so  that 
their  maximum  efficiency  occurs  at  about  three-quarters 
load.  This  is  especially  desirable  for  shop  work,  where 
the  driving  motors  are  called  upon  intermittently  to  give 
full  load,  the  average  demand  being  1 5  per  cent  to  30  per 
cent  less  than  the  load  for  which  they  are  rated. 

The  efficiency  of  a  10  H.P.,  60  cycle  motor  with  short- 
circuited  armature  is  shown  in  Fig.  59,  and  also,  for 
comparison,  the  curves  of  a  variable  resistance  high  in- 
ductance type.  The  efficiency  of  the  variable  resistance 
motor  is  the  higher  at  all  loads  under  full  load,  after 


86 


POLYPHASE  APPARATUS   AND   SYSTEMS. 


which  the  other  motor  is  ahead.  The  break-down  point 
of  the  latter  motor  is  over  200  per  cent  of  full  load. 

Condensers Condensers    are    used    to    improve    the 

power  factor  of  circuits  supplying  current  to  motors  by 
making  the  motors  take  current  in  proportion  to  the  loads. 
The  motors  themselves  are  not  improved,  but  the  wattless 
current  is  offset  by  the  leading  current  supplied  by  the 


Compensator  Type  Motor 

Hiah  Inductance  » 


40      60      80     100    120    140     160    130    200    220 

Per  Cent.  Load 
Fig.  59. 

condensers,  and  its  pernicious  influence  confined  to  the 
local  circuit  between  the  condenser  and  the  motor.  Fig. 
60  shows  the  apparent  efficiency  of  a  Stanley  two-phase 
motor  with  and  without  a  condenser,  and  Fig.  61  the  con- 
nection of  motor  and  condensers. 

The  condenser  consists  of  numerous  thin  sheet  conduc- 
tors, separated  by  still  thinner  dialectrics,  the  whole  elec- 
trically connected  to  form  two  conductors.  As  the  size 
of  the  condenser  increases  rapidly  with  a  low  frequency 


INDUCTION   MOTORS.  87 

and  voltage,  it  is  best  adapted  for  circuits  of  over  100 
cycles,  and  when  motors  are  used  for  not  less  than  500 
volt  circuits. 

EFFICIENCY  AND  POWER  FACTOR 

8       *        8        8        3 


Single-Phase  Motors Single-phase    induction    motors 

have   only  recently  been   commercially  introduced   on   a 
large  scale.     They  have  the  characteristic  form  of  poly- 


88         POLYPHASE  APPARATUS   AND   SYSTEMS. 


Fig,  61. 


INDUCTION  MOTORS. 


89 


phase  motors.  As  the  flow  of  energy  in  the  single-phase 
system  is  not  continuous,  as  in  a  polyphase  system,  their 
capacity  is  less  than  a  polyphase  motor  of  same  dimensions. 
In  respect  to  torque,  power  factor  and  efficiency,  even  the 
best  commercial  motors  are  not  so  good  as  polyphase 
motors.  An  external  starting  arrangement,  sometimes 
called  a  "  phase-splitter,"  is  sometimes  used  with  these 
motors,  for  artificially  producing  a  torque  sufficient  to 
enable  them  to  start  from  rest  under  a  partial  load. 


The  winding  of  a  two-pole,  single-phase  motor  is  shown 
in  Fig.  62.  It  has  a  two-phase,  interlinked  winding,  the 
common  terminals  being  at  III.  If  two  currents,  having  a 
difference  in  phase,  are  introduced,  the  dead  point  common 
to  all  single-phase  motors  will  be  overcome,  and  the  ar- 
mature will  revolve.  The  displaced  phase  is  produced  by 
a  combined  resistance  and  impedance  coil,  the  outline  con- 
nections of  which  are  shown  in  Fig.  63.  a  and  b  are 
the  main  leads ;  c  is  a  lead  to  the  common  terminal  of  the 
motor  two-phase  winding.  R  is  a  resistance  and  L  a  chok- 
ing coil.  The  current  passing  through  R  will  differ  in 


90         POLYPHASE  APPARATUS  AND   SYSTEMS. 

phase  from  that  flowing  through  L,  and  the  motor  will 
start,  when  the  switch  is  thrown,  with  a  torque  dependent 
upon  the  phase  difference.  The  maximum  torque  will  be 
developed  when  the  currents  are  90°  apart.  This,  of 
course,  cannot  be  obtained  with  this  device.  By  replacing 
the  resistance  by  a  condenser,  a  phase  difference  of  90°  or 
over  can  be  obtained,  with  a  correspondingly  increased 
torque,  and  a  decreased  starting  current.  When  the  motor 
reaches  speed,  the  starting  coils  are  cut  out,  and  it  then 
runs  as  a  single-phase  motor. 

The  usual  form  of  motor  is  provided  with  a  starting 
device  that  gives  half-load  torque  at  about  150  per  cent  of 
fulWoad  current.  Full  load  torque  may  be  obtained  at 
somewhat  over  twice  full-load  current,  by  a  special  start- 
ing [device.  jThe  advantage  of  the  single-phase  induction 
motor  over  the  single-phase  synchronous  motor  lies  prin- 
cipally in  the  fact  that  the  latter  motor  is  liable  to  be 
thrown  out  of  step  by  any  fluctuation  in  the  generator 
speed.  The  synchronous  motor  is  fairly  efficient,  and  has 
a  power  factor  of  nearly  unity,  but  the  current  at  starting 
is  quite  out  of  proportion  to  the  torque. 

A  three-phase  induction  motor  will  give  about  40  per 
cent  of  its  output  when  used  single-phase.  A  two-phase 
motor  will  give  50  per  cent  of  its  two-phase  rating  under 
the  same  conditions.  The  same  motors  can  be  rewound  as 
single-phase  motors,  and  will  then  have  an  output  of  over 
75  per  cent  of  their  former  rating.  The  unaltered  two- 
phase  and  three-phase  motors  can,  however,  be  made  to 
yield,  on  a  single-phase  circuit,  about  75  per  cent  of  their 
rating  by  increasing  the  voltage  30  per  cent  above  that  for 
which  they  are  wound. 

In    Fig.  64   is   shown    the   connections   of   a   Wagner 


INDUCTION   MOTORS. 


Electric  Company's  self-starting,  single-phase  motor.  In 
starting,  the  armature  and  field  are  connected  in  series. 
On  attaining  full  speed,  this  connection  is  automatically 
broken  by  a  governing  device  within  the  armature.  Simul- 
taneously, the  armature  is  short-circuited  on  itself,  and  the 


Line 


5 

Double  Pole 
Fuse  Block 

tjll 

UJc 

Q 

j-^\ 

Double  Pole 

; 

M 

Knife  Switch 

i   i 

yi,  Arnf 

r                i 

.^^    —  -< 

Fig-.  64. 

field  remains  connected  across  the  line.     The  motor  then 
operates  as  a  simple  induction  motor. 

As  seen  in  the  diagram,  no  external  starting  device  is 
required,  there  being  only  two  wires  from  the  mains  to 
the  motor.  The  third  binding  post,  C,  is  for  use  in  case 
the  voltage  of  the  supplying  circuit  is  low,  a  third  connec- 
tion and  a  double  throw  switch  being  required. 


92         POLYPHASE  APPARATUS  AND   SYSTEMS. 


CHAPTER   V. 
SYNCHRONOUS  MOTORS. 

General. — Any  alternating-current  generator,  with  little 
or  no  change,  can  be  used  as  a  synchronous  motor.  Elec- 
trically and  mechanically  the  motor  resembles  the  corre- 
sponding generator,  and  must  be  provided  with  the  same 
station  equipment,  including  some  source  of  exciting  cur- 
rent. The  synchronous  motor,  especially  in  units  of  large 
output,  possesses  a  number  of  features  which  makes  its 
use  at  times  preferable  to  that  of  the  induction  motor. 
Besides  the  advantage  of  an  unvarying  speed  at  all  loads, 
the  power  factor  can  be  altered  at  will  by  changing  the 
exciting  current  and  made  equal  to  unity  at  any  load. 
The  current  can  even  be  made  leading,  to  offset  a  lagging 
current  in  other  parts  of  the  system.  The  synchronous 
motor,  especially  at  low  speeds,  is  cheaper  to  build  than 
the  induction  motor,  and  its  efficiency,  as  a  rule,  will  be 
found  to  be  higher. 

As  a  partial  offset  to  these  advantages,  the  synchronous 
motor  is  not  adapted  for  use  where  a  large  starting  torque 
or  frequent  starting  of  the  load  is  necessary.  It  does  not 
admit  of  independent  speed  regulation.  It  also  has  the 
disadvantage  of  requiring  certain  station  appliances  and 
an  exciting  current,  which  is  usually  obtained  from  some 
source  other  than  the  motor. 

Speed.  —  The  speed  of  the  synchronous  motor  is  not 


SYNCHRONOUS   MOTORS.  93 

necessarily  the  generator  speed,  but  a  speed  which,  multi- 
plied by  the  number  of  poles,  gives  a  product  equal  to  the 
generator  alternations.  A  motor,  having  twice  the  number 
of  poles  that  the  generator  has,  will  have  half  the  speed, 
or  vice  versa.  As  load  is  thrown  on  the  synchronous 
motor,  there  is  a  lag  in  the  relative  positions  of  armature 
winding  and  pole  face,  or  retardation  of  the  armature. 
The  effective  counter  E.M.F.  is  thereby  reduced,  which 
gives  rise  to  a  larger  flow  of  current. 

The  motor  speed  is  independent  of  the  voltage  and  can- 
not be  altered  except  by  changing  the  generator  speed. 
It  is  important  therefore  that  the  regulation  of  the  prime 
mover  be  as  perfect  as  possible,  both  in  the  number  of  rev- 
olutions per  minute  and  in  the  angular  speed;  otherwise, 
as  the  fly-wheel  capacity  of  the  motor  armature  is  sufficient 
to  absorb  considerable  energy  without  changing  its  speed, 
fluctuating  currents  will  pass  between  generator  and  motor, 
reducing  the  motor  capacity,  and  producing  bad  regulation. 

Torque  and  Output.  —  A  synchronous  motor  at  starting 
acts  somewhat  as  an  induction  motor.  Consequently  any 
variation  of  its  proportions,  such  as  the  shape  of  the  pole 
pieces,  armature  reaction,  and  nature  of  the  winding,  —  i.  e., 
distributed  or  unitooth, — affects  its  starting  torque.  The 
starting  torque  may  vary  from  nothing  to  20  or  30  per 
cent  of  full-load  running  torque,  depending  upon  the  motor 
design.  When  once  in  motion,  the  motor  will  rapidly 
attain  synchronous  speed.  Polyphase  motors,  as  usually 
constructed,  will  carry  four  to  five  times  full  load.  If 
further  loaded,  they  fall  out  of  synchronism,  and  can  be 
brought  up  to  speed  by  being  relieved  of  the  load.  Single- 
phase  synchronous  motors  have  dead  points,  and  will  not 
start  from  rest;  monocyclic  generators  used  as  motors 


94    POLYPHASE  APPARATUS  AND  SYSTEMS. 

develop  too  feeble  a  torque  to  start,  and  may  be  regarded 
as  single-phase  motors  in  their  action  at  starting,  and  when 
running  under  load.  It  is  necessary  to  use  some  extra- 
neous source  of  power  to  start  single-phase  motors,  and 
bring  them  up  to  speed.  This  is  usually  effected  by  an 
alternating-current  motor.  In  some  cases  where  a  direct- 
current  source  of  power  is  available,  the  exciter  may  be 
used  as  a  starting  motor. 

The  limit  to  the  torque  and  output  of  a  synchronous 
motor  is  dependent  mainly  upon  the  terminal  voltage. 
Under  rated  voltage  the  margin  of  most  motors,  before 
the  break-down  point  is  reached,  is  sufficient  to  enable 
them  to  stand  a  heavy  overload.  Variation  of  the  speed 
of  the  prime  mover  will  reduce  the  maximum  output. 

Voltage The  relation  of  impressed  volts  to  the  max- 
imum output  is  the  same  in  synchronous  as  in  induction 
motors,  the  output  and  the  starting  torque  varying  within 
certain  limits  as  the  square  of  the  volts.  It  is  essential, 
therefore,  that  the  pressure  be  kept  at  the  rated  voltage  of 
the  motor;  otherwise  the  motor  may  not  start  at  all,  par- 
ticularly if  it  consumes  an  excessive  starting  current. 

Synchronous  motors  can  be  wound  for  the  same  voltage 
as  the  corresponding  generators.  Standard  motors  of  100 
H.P.  and  over,  of  the  revolving  armature  type,  are  wound 
for  potentials  up  to  three  3,400  volts.  Motors  of  the 
stationary  armature  type  can  be  safely  wound  for  poten- 
tials as  high  as  7,000  volts,  in  sizes  from  100  to  500  K.W.; 
motors  of  larger  capacity  can  be  wound  for  12,000  volts. 
Motors  of  the  revolving  field  type,  as  ordinarily  propor- 
tioned, have  a  somewhat  greater  starting  torque  than  those 
of  the  revolving  armature  type,  on  account  of  the  greater 
arc  covered  by  the  pole  face. 


SYNCHRONOUS    MOTORS.  95 

Methods  of  Starting When  a  large  torque  is  required 

to  turn  over  the  load,  as  in  the  case  of  mill  machinery  or 
long  lines  of  shafting  and  belting,  a  friction  clutch  must 
be  used.  This  permits  the  load  to  be  gradually  thrown  on 
the  motor  after  it  reaches  synchronism.  The  clutch  may 
be  mounted  on  the  motor  base  extended,  an  extra  standard 
being  required  for  this  purpose ;  or  the  motor  may  be 
belted  to  a  line  shaft  on  which  there  is  a  coupling.  This 
is  the  cheaper  and  more  usual  method.  Fig.  65  shows  a 
500  H.P.  motor,  built  with  extended  base,  carrying  a  clutch 
and  driving  pulley.  In  selecting  a  coupling  for  this  class 
of  work,  one  of  ample  proportions  should  be  used,  as  it 
must  start  the  load  gradually,  without  exceeding  the  break- 
down point  of  the  motor,  and  without  overheating. 

The  operations  in  starting  a  synchronous  motor  are 
about  as  follows :  First,  the  main  switch  is  closed  and  the 
motor  with  its  fields  unexcited  will  start  with  a  small 
torque  due  to  the  induced  currents  in  the  pole  pieces,  and 
soon  speed  up  to  almost  synchronism.  The  current  from 
the  exciter,  which  is  either  belted  to  or  mounted  on  the 
motor  shaft,  can  now  be  switched  into  the  fields,  and  the 
motor  will  be  brought  up  to  synchronism.  The  full  load 
can  then  safely  be  thrown  on  the  motor  by  the  friction 
clutch,  if  one  is  used. 

The  current  taken  at  starting  may  be  anything  from  150 
per  cent  of  full-load  current  to  several  times  normal  cur- 
rent, being  limited  by  the  resistance  and  self-induction  of 
the  armature  windings,  i.  e.,  its  impedance.  This  exces- 
sive starting  current,  as  it  is  of  an  inductive  character,  may 
cause  a  large  drop  in  the  line. 

If  the  motor  takes  a  large  proportion  of  the  generator 
output,  or  is  used  in  connection  with  lights,  and  started  and 


96         POLYPHASE  APPARATUS  AND   SYSTEMS. 


SYNCHRONOUS   MOTORS. 


97 


stopped  at  frequent  intervals,  Some  other  means  should  be 
employed  to  reduce  the  current.  This  can  be  done,  as  in 
the  case  of  the  induction  motor,  by  the  use  of  a  resistance, 
a  reactance,  or  a  compensator  in  the  main  circuit.  A  com- 
pensator starter,  like  that  shown  in  Fig.  66,  is  sometimes 
used.  This  par- 

Double-throw  Switch 


Running  tide- 


ticular  starting 
device  closely 
resembles  the 
compensator 
starter  for  three- 
phase  induction 
motors,  shown 
in  Fig.  50.  It 
is  provided  with 
three  taps,  giv- 
ing voltages  40 
per  cent,  50  per 
cent,  and  60  per 
cent  of  running 
full-load  voltage. 
With  50  per 

cent  of  the  impressed  volts,  the  synchronous  motor,  when 
properly  proportioned,  will  take,  at  starting,  a  current 
equal  to  about  full-load  current,  and  start  with  a  torque 
about  15  per  cent  of  the  full-load  running  torque.  The 
operation  of  this  starting  device  is  plainly  indicated.  The 
triple  pole  switch  is  down  at  the  moment  of  starting,  and, 
when  nearly  synchronous  speed  is  reached,  is  thrown  up  to 
the  running  side. 

The  current  may  also  be  reduced  by  means  of  a  starting 
motor,  usually  of  the  induction  type,  either  single-phase  or 


Continuous  Winding 

Fig.  66. 


98          POLYPHASE  APPARATUS  AND   SYSTEMS. 

polyphase.  The  current  taken  by  this  motor  is  too  small 
to  seriously  affect  the  voltage  of  the  circuit.  This  method 
should  be  employed  when  the  motor  is  started  frequently, 
or  when  a  low  starting  current  is  essential  to  preserve 
good  regulation.  A  starting  motor,  one-tenth  the  capacity 
of  the  synchronous  motor,  will  be  found  of  sufficient  size 
to  meet  all  average  conditions.  When  an  auxiliary  motor 
is  used,  the  synchronous  motor  must  both  be  brought  up 
to  slightly  above  synchronous  speed,  and  the  speed  of  the 
motor  E.M.F.  brought  into  opposition  with  the  generator 
E.M.F. 

Many  forms  of  self-starting  synchronous  motors  have 
been  devised  for  use  on  single-phase  circuits.  Most  of 
these  are  provided  with  a  commutator  for  self-excitation, 
and  a  starting  device.  A  commutator,  in  series  with  the 
field  winding,  rectifies  the  current  at  the  instant  the  main 
armature  current  is  in  phase  for  producing  a  slight  torque. 
When  the  motor  reaches  speed,  the  commutator  is  cut  out. 
One  of  these  types  is  the  single-phase  motor  made  by  the 
Fort  Wayne  Company,  which  embodies  a  modification  of 
this  construction.  The  main  current  is  first  thrown  on  a 
continuous  current  winding  connected  to  a  commutator, 
and  laid  over  the  alternating-current  winding  on  the  arma- 
ture, which  is  connected  to  collector  rings.  When  the  motor 
reaches  synchronism,  the  main  current  is  switched  into  the 
alternating-current  winding,  and  the  field  circuit  closed  on 
the  starting  winding  through  the  commutator. 

In  starting  a  synchronous  motor,  difficulty  is  sometimes 
encountered  in  the  high  voltage  induced  in  the  fields  by 
the  armature  current.  This  is  overcome  in  the  revolving 
field  type  of  motor  by  using  an  exciting  current  of  low 
potential,  —  sometimes  as  low  as  50  volts,  and  in  the  re- 


SYNCHRONOUS  MOTORS. 


99 


volving  armature  type  by  breaking  up  the  fields  into  a 
number  of  parts,  or  by  open-circuiting  each  field  spool,  as 
shown  in  Fig.  67.  Leads  from  each  spool  are  brought  out 
to  convenient  switches  on  the  motor  frame.  The  motor  is 
started  with  these  open.  When  synchronism  is  reached 
the  switches  are  closed,  thus  putting  the  field  coils  in  series, 
and  throwing  them  in  circuit  with  the  exciter. 


Fig.  67. 


Field  Excitation.  —  An  increase  of  the  field  excitation 
of  the  synchronous  motor  will  cause  a  corresponding  in- 
crease in  the  E.M.F.  generated  in  the  motor.  By  properly 
proportioning  the  field  excitation,  this  E.M.F.  of  the  motor 
can  be  made  considerably  greater  than  the  impressed  volts 
at  the  motor  terminals.  It  will  be  seen  that  an  opposite 
condition  exists  from  that  when  the  induced  E.M.F.  is 
small,  due  to  a  small  exciting  current.  In  the  first  case, 
the  phase  of  the  current  will  be  found  to  be  in  advance  of 
the  impressed  volts,  and  in  the  second  case,  to  be  lagging 
behind.  It  follows,  then,  that  for  any  condition  of  load  of 
the  synchronous  motor,  by  simply  changing  the  strength 


100   POLYPHASE  APPARATUS  AND  SYSTEMS. 


of  the  exciting  current,  the  armature  current  can  be  made 
lagging,  in  phase  with,  or  in  advance  of,  the  impressed 
E.M.F.  In  other  words,  the  amount  of  current  consumed 
by  the  motor  depends  upon  the  field  excitation. 

The  effect  upon  the  armature  current,  produced  by  vary- 
ing the  field  excitation,  is  shown  by  the  curves  in  Fig.  68. 
Up  to  a  certain  point,  as  the  excitation  is  increased,  the  ar- 
mature current  is  lagging,  and  decreases.  Further  increase 
of  the  exciting  current  causes  the  armature  to  consume  more 


Exciting  Current 
Fig.  68. 

current,  which  is  now  leading.  There  is  one  value  of  the 
exciting  current  for  which  the  armature  current  is  a  mini- 
mum. In  motors  of  good  regulation  this  value  varies  but 
slightly  with  different  loads. 

The  result  obtained  from  this  property  of  the  synchron- 
ous motor,  of  producing  at  will  any  displacement  of  phase 
between  current  and  E.M.F.,  is  the  possibility  of  annulling 
the  reactance  due  to  the  inductance  of  the  line,  and  at  the 
same  time  compensating  for  a  certain  amount  of  lagging 
current  due  to  inductive  loads  in  other  parts  of  the  circuit. 


SYNCHRONOUS   MOTORS. 


101 


When  over-excited,  the  synchronous  motor  acts  like  a 
great  condenser.  It  will  take  care  of  a  total  current 
made  up  of  energy  and  wattless  components,  to  an  ex- 
tent equal  to  its  rated  ampere  output. 

Synchronous  motors  of  the  polyphase  type  are  separately 
excited.  No  series  winding  or  automatic  compounding  is 
required. 


Exciting  Current  and  Exciters  for  Standard  Polyphase  Genera- 
tors and  Synchronous  Motors. 


GENERATOR  OR  MOTOR. 
RATING  K.  W. 

25-60  CYCLES. 

EXCITER 
CAPACITY. 

SEPARATE 
EXCITING 
CURRENT. 

VOLTAGB. 

(  Generator  .  . 
I  Motor    .... 

6 

10 

125 
I25 

1%  K.W. 
2      K.W.  to  2%  K.W. 

(  Generator  .  . 
'    I  Motor    .... 

8 
13 

I25 
125 

2     K.W.  to  2%  K.W. 
2%  K.W. 

ioc-|^enerator  '  ' 
I  Motor    .... 

10 

15 

I25 
I25 

254  K.W. 
2*  K.W. 

ISO_J  Generator  •  ' 
1  Motor    .... 

12 

2O 

I25 
125 

2%  K.W. 
3%  K.W.  to  4%  K.W. 

2,<^  5  Generator    '    ' 

i  Motor    .... 

19 
32 

125 
125 

4%  K.W. 
7%  K.W. 

When  generators  provided  with  automatic  compounding 
are  converted  into  synchronous  motors,  it  will  be  found 
that  increased  separate  excitation  is  required ;  since  the 
series  fields  are  necessarily  omitted  in  the  motor.  The 
standard  exciters  usually  furnished  with  generators  under 
250  K.W.  capacity  must,  as  a  rule,  be  replaced  by  the 
next  larger  sizes.  The  preceding  table  gives  the  average 
separate  exciting  current  required  by  standard  polyphase 
generators  and  motors  of  moderate  output,  power  factor 


102       POLYPHASE  APPARATUS   AND   SYSTEMS. 

being  taken  as  unity.  The  exciter  capacities  given  are 
sufficient  to  take  care  of  inductive  conditions.  The  ex- 
citers should  have  more  capacity  than  is  actually  required, 
as  they  are  the  weaker  part  of  the  system,  and  should  not 
be  taxed  to  their  full  capacity.  In  a  station  where  several 
synchronous  motors  or  generators  are  used,  it  is  customary 
to  install  two  exciting  dynamos,  each  one  of  which  has 
capacity  to  furnish  sufficient  excitation  for  all  machines. 

Power  Factor.  —  The  maximum  efficiency  of  the  motor 
and  circuit  exists  when  the  current  and  E.M.F.  supplied 
to  the  motor  are  in  phase,  —  i.e.,  when  the  power  factor  is 
unity.  This  is  also  a  condition  of  minimum  current,  and 
the  drop  in  the  line  is  that  due  to  ohmic  resistance  only. 
When  the  current  is  in  advance  of,  or  lagging  behind  the 
impressed  volts,  the  power  factor  is  less  than  unity.  It  is 
possible,  as  we  have  seen,  to  suitably  proportion  the  excit- 
ing current  of  a  synchronous  motor,  so  that  its  power 
factor  may  be  unity  at  any  load.  In  this  way  a  low  power 
factor  of  the  supplying  circuit,  due  to  induction  motors, 
may  be  raised  any  amount. 

On  account  of  armature  reaction,  a  motor,  which  has  its 
excitation  adjusted  to  give  a  power  factor  of  unity  at  full 
load,  will  take  a  leading  current,  and  have  a  power  factor 
less  than  100  per  cent  at  all  points  below  full  load.  For 
the  average  case,  it  will  be  found  most  desirable  to  so 
excite  the  motor  fields  that  the  minimum  current  and 
highest  power  factor  are  reached  at  about  average  load. 
The  power  factor  will  be  leading  at  lighter,  and  lagging  at 
greater,  loads.  Except  in  the  case  of  synchronous  motors 
of  abnormally  bad  design,  the  power  factor,  with  properly 
excited  fields,  will  have  a  high  value  over  a  wide  range  of 
load.  Even  motors  with  considerable  armature  reaction 
will  have  only  a  slightly  drooping  curve  of  power  factor. 


SYNCHRONOUS   MOTORS.  IO3 

Motors  which,  as  generators,  would  have  excellent  in- 
herent regulation,  —  i.e.,  small  armature  reaction  and  self- 
induction,  —  can  be  made  to  have,  with  one  adjustment  of 
the  field,  practically  100  per  cent  power  factor  at  all  but 
light  loads.  The  advantage  of  a  more  uniform  power 
factor  in  such  motors  is  offset  by  their  instability  during 
voltage  fluctuations.  Some  self-induction  is  desirable  in 
order  to  prevent  exchange  of  current  between  motor  and 
lines  when  the  impressed  volts  vary,  as  often  happens  in 
power  transmissions.  In  selecting  a  synchronous  motor, 
therefore,  preference  should  be  given  to  that  one  which,  as 
a  generator,  would  not  have  very  close  inherent  regulation. 
Machines,  of  not  such  good  regulation,  have,  as  a  rule,  a 
higher  efficiency,  and  take  less  starting  current. 

To  predetermine  the  proper  field  strength  which  will 
give  the  maximum  condition  of  efficiency,  it  is  necessary 
to  know  the  conditions  of  the  system,  —  the  reactance  of 
the  generator  and  line,  the  average  load  and  its  power  fac- 
tor, and  the  characteristics  of  the  motor.  Each  case  is  a 
problem  by  itself,  and  must  be  judged  by  the  special  con- 
ditions affecting  it. 

A  synchronous  motor  will  take  no  more  than  its  rated 
amperes  without  overheating,  whatever  the  phase  relation 
of  current  and  E.M.F.  may  be.  If  the  inductive  load  at 
the  receiving  end  is  large,  as  compared  with  the  motor 
load,  the  synchronous  motor  may  prove  inadequate  to  carry 
its  own  load,  and  appreciably  annul  the  inductive  effects. 

It  will  be  found  that,  for  every  load  and  every  power 
factor,  there  is  a  synchronous  motor  capacity  which  will 
make  the  efficiency  of  the  system  a  maximum.  Mr.  E.  J. 
Berg  has  calculated  the  influence  of  synchronous  motors 
upon  the  efficiency  of  alternating  systems.  Fig.  69  shows 


104   POLYPHASE  APPARATUS  AND  SYSTEMS. 

the  different  efficiencies  of  a  transmission  of  a  constant 
current  of  200  amperes  when  a  synchronous  motor  of  50, 
100,  or  150  K.W.  is  running  as  a  compensator  at  the 
receiving  end,  which  is  assumed  to  have  varying  power 


I/A  E  EFFICIL  NCY  CUR  I ES 


flO          80          70  60          50         40  30          20          10 

POWER  FACTOR  (RECEIVING  END)  ' 
Pig.  69. 

factors.     The  circuit   is   supposed  to  have   the   following 
constants : — 

Current  =200  amperes. 

Resistance  =  .52  ohms. 

Reactance  =  1.45  ohms. 

Voltage  at  motor  =  1000. 

It  will  be  noticed  that,  as  the  power  factor  diminishes  at 
the  receiving  end,  the  line  efficiency  is  increased  by  using 
the  larger  synchronous  motors  —  i.e.,  will  transmit  a  great 


SYNCHRONOUS   MOTORS.  10$ 

amount  of  energy  for  the  same  loss.  The  line  efficiency  is 
greatest  when  using  the  150  K.W.  motor  at  all  power 
factors  below  87.  The  line  efficiency  is  improved  by  en- 
tirely dispensing  with  the  motors  when  the  power  factor  is 
greater  than  95.  The  leading  current  of  the  motors  is 
then  in  excess  of  the  lagging  current  of  the  receiving  cir- 
cuit, thereby  increasing  the  total  current,  or  when  main- 
taining a  constant  current,  as  in  the  present  case,  decreasing 
the  energy  current  —  i.e.,  the  amount  of  power  that  can  be 
transmitted  over  the  lines  with  the  conditions  as  given. 


106       POLYPHASE   APPARATUS   AND   SYSTEMS. 


CHAPTER    VI. 
ROTARY   CONVERTERS. 

General.  —  Any  direct-current  generator  can  be  used  as 
a  rotary  converter  by  tapping  the  armature  windings  at 
particular  points  and  connecting  the  leads  to  collector 
rings.  Direct  current  can  be  taken  from  the  brushes  of 
the  machine  at  the  commutator  end,  if  an  alternating  cur- 
rent is  supplied  to  the  -collector,  or  vice  versa.  If  connec- 
tions are  made  with  the  armature  at  points  differing  from 
each  other  by  180  electrical  degrees,  the  machine  becomes 
a  single-phase  rotary  converter  ;  while  connections  at  points 
90°  apart  will  give  a  two-phase  relationship.  Connections 
made  at  points  1 20  electrical  degrees  apart  permit  the  use 
of  the  machine  on  three-phase  circuits. 

The  output  of  such  a  machine  is  increased  when  used  as 
a  rotary  converter.  This  is  partly  due  to  the  absence  of 
armature  reaction.  The  direct  current  flowing  out  may 
be  said  to  neutralize  the  armature  reaction  of  the  alternat- 
ing current  flowing  in.  Again,  at  certain  positions  of  the 
armature,  the  current  flows  through  the  -shortest  possible 
path  from  collector  to  commutator.  When  used  as  a  motor, 
taking  current  from  either  the  direct  or  alternating  current 
end,  a  rotary  will  heat  more  for  the  same  current  input  than 
when  used  solely  for  the  conversion  of  the  current. 

While  a  direct-current  generator  may  be  made  into  a 
rotary  converter  in  the  manner  described,  it  is  not  desir- 


ROTARY    CONVERTERS.  IO/ 

able  to  do  so,  on  account  of  the  low  frequency  which  such 
a  machine  will  have.  It  does  not  follow,  moreover,  that 
the  direct-current  generator  will  fulfil  the  conditions  of 
successful  commercial  operation.  On  the  contrary,  it  is 
probable  that,  without  some  change  in  the  proportioning 
of  parts  and  windings,  such  a  rotary  converter  would  be  a 
failure. 

As  usually  designed,  rotary  converters  vary  but  little  in 
mechanical  construction  and  in  general  appearance  from 
direct-current  generators.  Fig.  70  illustrates  a  400  K.W. 
Westinghouse  converter.  This  machine  as  shown  is  pro- 
vided with  an  induction-starting  motor,  which  is  used  when 
the  converter  is  started  from  the  alternating-current  end, 
and,  like  the  similar  motor  in  a  synchronous  motor,  reduces 
the  starting  current.  No  pulley  is  provided,  unless  it  is 
intended  to  operate  the  rotary  as  a  double-end  generator  or 
as  a  motor.  The  armature  is  usually  of  large  diameter,  to 
give  efficient  ventilation.  That  of  a  600  K.W.  rotary, 
recently  built,  has  the  high  peripheral  speed  of  7,500  feet 
per  minute. 

Connections.  —  A  number  of  connections  of  the  alter- 
nating end  of  rotary  converters  are  diagramatically  shown 
in  Figs.  71  to  75.  Fig.  71  is  a  single-phase  arrangement. 
The  armature  windings  are  tapped  at  two  opposite  points, 
and  leads  are  brought  out  to  two  collector  rings.  The 
connections  for  three-phase  rotary  converters  are  shown  in 
Fig.  72.  The  three  collector  rings  are  connected  to  three 
points  in  the  armature,  120°  apart.  Fig.  73  illustrates  the 
usual  method  of  making  connections  for  a  two-phase  rotary 
converter.  These  are  the  simple  connections  of  bipolar 
machines.  In  multipolar  rotary  converters,  the  collector 
rings  are  connected  to  as  many  points  of  the  armature  as 


108        POLYPHASE   APPARATUS   AND    SYSTEMS. 


ROTARY   CONVERTERS. 


109 


there  are  pairs  of  poles,  —  i.e.,  the  connections  must  be 
duplicated  for  eachfc36o  electrical  degrees  of  the  machine. 
Fig.  74  shows  the  connections  of  a  four-pole  single-phase 
rotary.  The  two  pairs  of  leads  run  from  points  of  the 
armature  winding,  180  electrical  degrees  apart. 


Fig.  71. 


Fig.  73. 


Fig.  74. 


It  has  been  noticed  that  the  increased  output  of  a 
machine,  when  used  as  a  rotary  converter,  is  partly  due  to 
some  of  the  current  passing  directly  from  collectors  to 
commutator.  The  output  can  be  made  still  greater  by 
increasing  the  number  of  collector  rings  and  connections. 
For  instance,  a  three-phase  arrangement  can  be  made  with 


1 10    POLYPHASE  APPARATUS  AND  SYSTEMS. 


six  collector  rings,  as  in  Fig.  75,  and  a  two-phase,  with 
eight  collector  rings.  In  the  three-phase  arrangement,  the 
phases  are  not  interlinked  at  the  collector  rings. 

Ratio  of  Alternating  to  Direct-Current  Voltage.  —  The 
voltage  of  the  alternating  current  of  a  rotary  converter  is 
always  less  than  that  of  the  direct-current  end,  the  value 
of  which  is  equal  to  the  crest  of  the  E.M.F.  wave,  while 
the  alternating  pressure  is  rated  by  the  mean  effective 

value.  The  ratio  of  voltage 
for  any  particular  converter 
cannot  be  appreciably  va- 
ried. In  fact,  it  is  practi- 
cally unalterable,  except  by 
a  change  in  the  wave  shape 
of  the  E.M.F.  The  nat- 
ural E.M.F.  of  a  direct- 
current  generator  is  alterna- 
ting  in  character,  and  is 
rectified  by  the  commutator 
when  the  impulses  are  at  their  maximum.  The  measured 

or  effective  value  of  this  unrectified  E.M.F.  is  —~  —  .707 

"J  2 

of  the  E.M.F.,  at  the  commutator  brushes.  This  is  the  re- 
lation between  the  alternating  and  direct  volts  of  a  single- 
phase  and  of  a  two-phase  rotary  converter.  From  the 
nature  of  the  three-phase  system  of  electro-motive  forces, 
the  ratio  of  voltages  in  a  three-phase  rotary  converter  is 


75. 


._  =  .613  of  the  direct  current  E.M.F. 


Accordingly,  to 


obtain  a  direct  current  from  a  converter  of  600  volts,  alter- 
nating currents  of  375  to  420  volts  must  be  supplied  to  it. 
Step-down  transformers  are  consequently  used  in  all  cases 
where  the  primary  current  is  not  of  the  proper  voltage. 


ROTARY   CONVERTERS. 


Ill 


The  theoretical  ratios  are  not  always  found  in  practice, 
due  mainly  to  the  departure  of  the  generator  voltage  from 
a  true  sine-wave,  affecting  the  mean  value  of  the  alternating 
E.M.F.,  and  to  the  drop  in  the  machine,  which  may  be 
i  or  2  per  cent. 


22u 


2<V 


UO 


// 


3s 


s: 


MO 


\ 


^ 


5i 


o-^d 


A 


^ 


M 


40 


\A 


X 


\ 


M^l 


0          .2         .4          .6          .8         1.0        1.2        1.4        Lti        L«        2.0        2.2 

AMPERES  FIELD 
Pig.  76. 

The  divergence  from  the  theoretical  ratio  in  rotary  con- 
verters is  rarely  more  than  6  or  7  per  cent. 

Types  of  Converters  determined  by  Field  Excitation — 
Rotary  converters  may  be  either  separately  excited  or 
have  both  series  and  separate  field  excitation, —  i.e.,  be 
shunt  or  compound  wound.  A  third  type  is  sometimes 
constructed,  which  has  neither  separately  nor  series  excited 


112   POLYPHASE  APPARATUS  AND  SYSTEMS. 

fields,  but  in  which  the  magnetic  field  is  induced  by  the 
armature  current.  This  type  is  known  as  the  "  Induction  " 
converter,  and  has  the  characteristic  of  an  induction  mo- 
tor, of  a  lagging  current  at  all  loads.  It  runs,  however,  at 
a  synchronous  speed. 

The  current  for  the  separately  excited  fields  is  usually 
supplied  from  the  direct-current  end,  so  no  exciter  is  re- 
quired. The  shunt-wound  rotary  can  be  made  to  give 


uuuu 

2300 
2000 
1600 
1000 
600 

( 

.  \   - 

55C 

Volts  L 

.  C  ci 

nstant 

Potential 

)    80    40    60    80   100   130   140   160   180   200   22 

A  m per os  D,  0. 

Fig.  77. 

any  power  factor,  either  leading  or  lagging,  by  either  over 
or  under  exciting  the  fields.  The  power  factor  will  re- 
main practically  constant  for  all  loads.  This  property  is 
graphically  shown  in  Fig.  76.  Each  curve  represents  the 
variation  in  the  current  input  at  the  alternating  end,  for 
varying  field  strengths  at  different  loads,  of  a  100  K.W. 
rotary  converter.  The  field  strength  for  minimum  current, 
or  100  per  cent  power  factor,  is  9.2  amperes  at  no  load, 
and  9.55  amperes  at  full  load  of  182  amperes,  proving  that 
the  armature  reaction  is  very  slight. 


ROTARY   CONVERTERS.  113 

Fig.  77  shows,  in  another  way,  the  insignificance  of  the 
armature  reaction.  The  ampere  turns  at  no  load  are  2,700, 
and  at  full  load  2,790,  an  increase  of  about  3  per  cent. 

The  shunt-wound  converter  is  particularly  adapted  for 
lighting  and  other  steady  loads,  where  good  regulation 
and  constant  power  factor  are  of  importance. 

Compound-wound  converters  are  used  to  advantage,  as 
will  be  shown,  for  supplying  current  to  fluctuating  cir- 
cuits, as  in  railway  service,  and  in  cases  where  it  is  neces- 
sary to  maintain  constant  or  increasing  voltage  with  in- 
creasing load.  Various  combinations  of  field  excitations 
are  possible,  and  more  or  less  prominence  can  be  given 
the  shunt  or  series  windings,  as  may  be  required. 

Limit  of  Frequency.  —  While  60  cycle  rotary  converters 
of  a  capacity  as  great  as  500  K.W.  have  been  built,  the 
greatest  success,  up  to  the  present  time,  has  undoubtedly 
been  obtained  by  using  a  frequency  of  approximately  40 
cycles  and  under.  The  limit  of  frequency  of  a  rotary  con- 
verter is  due  solely  to  mechanical  reasons.  In  designing 
a  machine  for  a  given  number  of  alternations,  the  problem 
is  to  keep  the  peripheral  speed  of  the  commutator  within 
practical  limits.  Too  high  a  peripheral  speed  will  cause 
the  commutator  segments  to  buckle,  through  the  action  of 
centrifugal  force.  A  reduction  in  the  diameter  of  the 
commutator,  on  the  other  hand,  may  reduce  the  width  of 
the  segments  below  the  lowest  limit  fixed  by  experience 
with  commutator  construction  and  operation.  The  volt- 
age and  output  will  determine  the  general  dimensions  of 
the  commutator.  Take  the  case  of  a  600  K.W.,  550  volt, 
60  cycle  rotary  converter,  the  speed  of  which,  on  account 
of  its  size,  is  limited  to,  say,  600  R.P.M.  The  number  of 
poles  would  be  twelve.  The  peripheral  speed  of  the  com- 


114  POLYPHASE  APPARATUS  AND  SYSTEMS. 

mutator  being  limited,  the  circumference  is  at  once  fixed. 
The  average  volts  per  bar  being  also  limited,  the  total 
number  of  segments  is  determined.  In  a  40  cycle  rotary 
recently  constructed,  the  average  voltage  between  seg- 
ments was  limited  to  1 3^  volts,  and  the  commutator  speed 
to  4,500  feet  per  minute.  If  we  apply  this  data  to  the  60 
cycle  rotary,  we  have  the  following  : 

Number  of  segments  between  poles  =550-5-13!  =41 

Total  number  of  segments  =  12  (number  of  poles)  X  41  =492 

That  circumference  of  the  commutator  which  will  keep 
the  peripheral  speed  within  the  limits  set  —  i.e.,  4,500  feet 
per  minute — is  90",  thus  allowing  only  .18",  for  the  width 
of  each  segment.  For  mechanical  reasons  this  width  is 
less  than  can  be  used.  It  will  be  seen  that,  unless  the 
speed  of  the  rotary  can  be  increased,  thus  permitting  a 
lesser  number  of  poles,  or  the  peripheral  speed  of  the  com- 
mutator can  be  increased,  permitting  a  larger  circumfer- 
ence, and  consequently  wider  segments,  the  difficulty  can 
only  be  overcome  by  using  a  double  commutator.  This, 
however,  involves  a  complication  of  collector  rings  and 
connections,  and  the  current  must  be  commuted  twice 
and  the  commutator  losses  doubled.  This  rotary  could 
be  built  with  one  commutator,  if  wound  for  no  volts  or 
thereabouts.  The  general  statement  may  be  made  that, 
for  frequencies  over  35  to  40  cycles,  it  is  more  difficult  to 
build  rotaries  for  high  voltage  than  for  low  voltage,  —  i.e. 
for,  say,  555  volts, — than  for  100  to  200  volts,  but  not 
such  a  difficult  problem  to  wind  a  converter  of  under  35 
cycles  for  the  higher  voltage. 

Regulation  of  Voltage  by  Field  Excitation Like  the 

synchronous  motor,  the  rotary  converter  can  be  used  to 


ROTARY   CONVERTERS.  115 

annul  Self-induction  of  the  line  and  the  results  of  poor 
power  factors  of  other  parts  of  the  system.  For  purposes 
of  automatic  compounding,  the  shunt-wound  rotary  con- 
verter is  useless  on  account  of  its  constant  power  factor. 
The  compound  rotary,  however,  fulfils  the  exact  conditions 
required  for  overcoming  the  drop  in  line,  and  thereby  main- 
taining constant  voltage  at  the  direct-current  end,  or  for 
raising  the  alternating  voltage  with  increasing  load,  and, 
thereby,  the  direct-current  voltage.  This  regulation  can 
be  effected  without  any  change  in  the  generator  excitation 
simply  by  varying  the  phase  relationship  of  current  and 
volts. 

As  an  illustration  of  the  use  of  this  valuable  feature  of 
a  rotary  converter,  let  us  take  the  case  of  a  generator,  with 
constant  field  excitation,  supplying  current  to  a  converter  for 
street  railway  service,  over  transmission  lines  having  a 
reactance  and  resistance.  The  voltage  drop  is  still  further 
increased  at  full  load  by  the  reactance  df  generator  and 
converter. 

The  compound  field  of  the  rotary  is  proportioned  so  that 
at  no  load  it  is  underexcited.  The  E.M.F.  of  the  rotary  is 
then  considerably  less  than  the  impressed  E.M.F.,  and  cur- 
rent in  the  line  is  made  lagging.  The  E.M.F.  of  self- 
induction  is  thereby  increased  so  that  the  voltage  of  the 
system  is  cut  down,  giving  a  voltage  at  the  collector  rings 
corresponding  to  the  500  volts  direct  current. 

As  the  load  increases,  the  excitation  is  increased  by  the 
series  fields,  thereby  increasing  the  rotary  E.M.F.,  and  at 
some  intermediate  point  bringing  current  and  E.M.F.  in 
phase.  The  drop  of  voltage  is  then  due  to  resistance 
only.  At  full  load  the  converter  is  overexcited,  and  the 
rotary  E.M.F.  is  greater  than  the  impressed.  The  current 


Il6     POLYPHASE  APPARATUS  AND   SYSTEMS. 

is  then  leading,  and  the  voltage  is  actually  higher  at  the 
converter  than  at  the  generator.  In  this  way  the  pressure 
at  the  commutator  of  the  rotary  is  made  550  volts. 

The  excitation  can  be  adjusted  so  as  to  maintain  con- 
stant voltage  at  the  commutator  brushes,  the  automatic 
regulation  taking  care  of  line  and  converter  drop  only. 

For  any  particular  over-compounding  or  compensation  of 
voltage  drop,  a  certain  amount  of  self-induction  must  be 
present  in  the  system.  The  best  results  in  compounding 
are  obtained  when  the  rotary  is  operated  from  its  own  inde- 
pendent circuit,  and  when  generator,  line,  and  converter  are 
carefully  adjusted  for  the  compounding  required.  This  ad- 
justment not  infrequently  includes  an  artificial  reactance 
such  as  a  choking  coil. 

A  graphical  demonstration  of  the  variation  of  voltage 
due  to  power  factors,  both  lagging  and  leading,  is  given  in 
Chapter  I. 

Power  Factor.  —  The  power  factor  of  the  compound- 
wound  rotary  converter  excited  for  unit  power  factor  at 
full  load  is  not  so  good  at  light  loads.  The  power  factor 
of  the  shunt-wound  converter  we  have  seen  is  the  same  at 
all  loads.  The  induction  type  has  a  variable  power  factor 
which  is  not  so  good  as  that  of  the  compound  rotary. 

A  variation  of  the  reactance  in  the  supplying  circuit 
will  change  the  curve  of  power  factor  for  various  loads. 
This  is  due  to  the  fact  that  the  field  excitation  must  be 
increased,  or  reduced,  as  the  case  may  be,  in  order  to  main- 
tain a  100  per  cent  power  factor  at  any  predetermined 
percentage  of  load.  To  obtain  10  per  cent  over-com- 
pounding, the  fields  of  the  compound  rotary  converter  are 
excited  to  give  a  power  factor  of  unity  at  usually  f  load ; 
and  when  it  is  desired  to  maintain  a  constant  voltage  at 


ROTARY   CONVERTERS. 


117 


the  commutator,  the  fields  are  ordinarily  adjusted  to  give 
this  power  factor  at  full  load.  Mr.  E.  J.  Berg,  who  has 
given  much  study  to  the  practical  application  of  these 
principles,  has  calculated  some  power-factor  curves  which 
illustrate  the  amount  of  reactance  necessary  to  effect  a 
compounding  of  the  direct  current  in  a  rotary  converter 
to  which  current  is  supplied  by  its  own  transmission  line. 
In  Fig.  78,  curve  2  is  the  curve  of  power  factor  of  a  series- 
wound  rotary  when  excited  to  have  a  power  factor  of  unity 


0    J    JL  .3   .4    .5    .6   .7    .8    .9   1.01.1  1.2  L8 1.4  U  1.6  1.7  1.8  1.9  2.0 
Output  from  Continuous  Current  Side  of  Rotary 
Fig.  78, 

at  f  load.  The  reactance  of  the  generator,  line,  and  con- 
verter is  assumed  as  40  per  cent ;  the  resistance  as  10  per 
cent ;  the  generator  excitation  is  also  assumed  as  constant 
under  these  conditions.  The  power  factor  at  full  load  is 
98^  per  cent;  at  f  load,  100  per  cent;  -J-  load,  97^  per 
cent ;  i  load,  79  per  cent ;  and  at  y1^  load,  47  per  cent. 
Mr.  Berg  then  assumes  that,  instead  of  constant  field  exci- 
tation of  the  generator,  the  terminal  voltage  is  kept  con- 
stant. This  would  correspond  to  a  case  where  the  rotary 
transmission  lines  were  fed  from  the  station  bus-bars.  The 
total  reactance  of  the  system  is  then  reduced  by  that  of  the 


Il8   POLYPHASE  APPARATUS  AND  SYSTEMS. 

generator,  becoming  10  per  cent.  Curve  3  shows  the  power 
factor  at  all  loads  for  these  conditions.  It  is  necessary  to 
reduce  the  excitation  in  order  to  maintain  the  power  factor 
unity  at  f  load.  The  power  factor  is  much  lower  at  other 
loads,  and  the  condition  of  operation  is  by  no  means  as  sat- 
isfactory as  before.  The  plant  can  be  made  so  by  introdu- 
cing in  the  line  an  external  reactance  equal  to  the  generator 
reactance.  This  may  be  any  form  of  a  choking  coil.  The 
power-factor  curves  at  the  generator  terminals,  with  the 
former  constants,  are  plotted  in  the  figure  as  curve  I. 

The  power  factors  of  an  induction  converter  of  600 
K.W,  capacity  are  as  follows  : 

Full  load 91  per  cent 

f  load 87  per  cent 

%  load 77  per  cent 

Starting  of  Rotary  Converters.  —  Self-starting  rotary 
converters  are  set  in  operation  by  introducing  either  alter- 
nating current  to  the  collector  rings,  or  direct  current  to 
the  commutator.  When  starting  from  the  alternating-cur- 
rent end,  the  fields  should  not  be  excited.  The  starting 
current  in  a  well-designed  rotary  is  rarely  more  than  50 
per  cent  greater  than  normal  full-load  current.  This  can 
of  course  be  reduced  by  the  same  means  employed  in  the 
starting  of  synchronous  motors.  The  rotary  converter  is 
started  from  the  direct-current  end  in  the  same  way  as  a 
shunt-wound  direct-current  motor.  The  fields  should  be 
fully  excited,  and  there  should  be  a  resistance  in  series  with 
the  armature  when  the  motor  switch  is  closed.  Failure  to 
excite  the  field  may  cause  the  rotary  to  race  like  any  shunt 
motor.  Converters  are  also  frequently  started  by  auxiliary 
motors. 


ROTARY   CONVERTERS.  119 

Rotary  converters  can  be  run  in  parallel  either  on  the 
direct-current  or  the  alternating-current  ends.  When  two 
or  more  rotaries  are  to  be  run  together,  they  can  be  brought 
into  synchronism  by  the  same  method  as  in  the  practice 
with  alternating-current  generators.  After  the  main  switch 
is  closed,  the  field  switch  is  then  closed,  if  the  rotary  has 
been  started  from  the  alternating-current  end.  When 
started  from  the  direct-current  end  the  machine  is  synchro- 
nized ;  then  the  field  switch,  which  supplied  excitation  for 
starting,  is  opened,  and  finally  the  switch,  supplying  its  own 
field,  is  closed. 

In  starting  a  self-exciting  or  shunt-wound  rotary  from 
the  alternating  side,  there  is  no  way  of  telling  whether  the 
polarity  will  be  positive  or  negative.  This  difficulty  may 
be  overcome  by  separately  exciting  the  machines. 

Equalizers  must  always  be  used  with  compound  type  of 
rotary  converters.  The  equalizing  switch  should  be  closed 
before  the  machines  are  thrown  together,  as  then  the  ro- 
tary will  always  maintain  the  same  polarity  at  the  commu- 
tator brushes  ;  otherwise,  if  the  series  field  predominates, 
the  current  may  be  so  far  in  advance  as  to  reverse  the 
polarity. 


120       POLYPHASE   APPARATUS   AND   SYSTEMS. 


CHAPTER    VII. 
STATIC  TRANSFORMERS. 

Polyphase  Transformers.  —  Transformers  for  use  on 
polyphase  circuits  may  be  either  of  a  compound  type, 
wound  polyphase,  or  plain  single-phase.  Many  European 
firms  manufacture  two  and  three-phase  transformers,  but 
this  practice  has  been  seldom  followed  in  this  country. 
Polyphase  transformers  usually  have  as  many  magnetic 
circuits  as  there  are  phases,  the  flux  in  which  follows  the 
same  course  as  the  flow  of  current  in  the  corresponding 
conductor  mesh.  The  iron,  therefore,  is  used  to  better 
advantage  than  in  separate  single-phase  transformers,  and 
less  is  required  for  the  same  output.  The  two-phase  trans- 
former is  sometimes  made  with  three  magnetic  circuits  and 
connected  on  the  three-wire,  two-phase  system.  Fig.  79 
shows  a  three-phase  transformer,  with  its  case  removed, 
made  by  the  Siemens-Halske  Company. 

American  engineers  use  an  appropriate  combination  of 
single-phase  transformers  for  all  the  commercial  polyphase 
systems.  Aside  from  the  simpler  construction  and  greater 
flexibility  of  the  single-phase  type,  this  arrangement  has  the 
advantage  of  not  being  rendered  entirely  inoperative  by 
damage  to  one  transformer.  The  remaining  uninjured 
transformer  or  transformers  can  frequently  maintain  con- 
tinuous, though  possibly  crippled,  service. 

The  growing  application  of  electricity  to  the  transmis- 


STATIC  TRANSFORMERS. 


121 


sion  of  power  over  long  distances,  and  the  increasing  size 
of  electrical  units,  has  necessitated  a  change  in  transformer 
construction.     The  radiating  surface  of  a  transformer  in- 
creases as  the  square  of  its   linear  dimensions,   while  its 
mass  varies  as 
the  cube  of  the 
dimensions. 
For  this  reason 
transformers  of 
a  certain  type 
of   moderate 
sizes  easily  re- 
main   cool    by 
self -radiation, 
but,  if  made  of 
greater  capaci- 
ties, would  burn 
out    unless 
cooled  by  some 
artificial  means. 
The  ordinary 
lighting    trans- 
former is  cooled 
by  being  im- 
mersed in    oil. 
The  heat  gene- 
rated  in  the 
coils    and    the 
iron  easily  finds  its  way  to  the  iron  casing,  and  is  thence 
dissipated  by  radiation.      Transformers  of   this  type  are 
rarely  built  of  larger  size  than  50  to  75   K.W.     Trans- 
formers of  greater  capacity  must  have  some  special  means 


Fig.  79. 


122       POLYPHASE   APPARATUS   AND    SYSTEMS. 


of  getting  rid  of  the  heat  generated  within  them.  A  num- 
ber of  methods  are  employed  for  cooling  transformers,  but 
all  may  be  classed  under  the  headings  of  self-cooled  and  of 
artificially  cooled  transformers.  It  will  be  more  satisfac- 
tory, however,  to  describe  the  various  types  under  their 
trade,  and  at  the  same  time  descriptive,  names. 

Self-Cooled  Oil  Transformers.  —  The    ordinary  lighting 
transformer  is  of  the  self-cooling  type.    The  magnetic  cir- 


Time  in  hours 

Fig,  80. 

cuit  is  usually  a  plain  rectangle  of  interlaced  strips  of  iron, 
permitting  a  simple  form  of  winding.  The  insulation  be- 
tween primary  and  secondary  is  tested  to  10,000  volts 
alternating.  A  twofold  advantage  is  gained  by  immersing 
these  transformers  in  oil :  First,  the  temperature  is  reduced 
by  offering  a  ready  means  of  escape  for  the  heat  ;  second, 
punctures  in  insulation  are  immediately  repaired  by  the 
inflow  of  the  oil. 

The  reduction  of  temperature  by  the  use  of  oil  is  shown 
in  Fig.  80.     Curve  I   gives  the  rise  in  temperature  of  a 


STATIC   TRANSFORMERS. 


123 


124   POLYPHASE  APPARATUS  AND  SYSTEMS. 

transformer  not  submerged  in  oil,  as  determined  by  the 
increase  of  resistance  method.  Curve  2  shows  the  tem- 
perature of  the  transformer  immersed  in  oil.  Curve  3  is 
the  temperature  of  the  oil.  Curve  4  is  the  temperature  of 
the  windings  of  another  transformer  of  poorer  design. 
Curve  5  shows  the  temperature  of  the  same  as  determined 
by  thermometer.  This  last  curve  does  not  give  the  true 
heating,  for  the  thermometer  cannot  reach  the  inaccessible 
portions  of  the  transformer.  These  transformers  cannot 
be  wound  for  higher  potentials  than  3,000  volts,  without 
a  serious  loss  in  capacity,  as  the  copper  must  be  sacrificed 
for  the  increased  thickness  of  insulating  material  required. 

Transformers  of  the  self-cooling  oil  type  for  high  vol- 
tages and  for  power  service  are  modified,  to  facilitate  the 
dissipation  of  the  heat  which,  in  the  larger  sizes,  could  not 
be  radiated  without  some  special  arrangement. 

Fig.  8 1  illustrates  a  number  of  this  type,  as  made  by  the 
Westinghouse  Electric  Company,  with  the  cases  removed. 
The  windings  are  divided  into  a  number  of  coils  which,  as 
will  be  seen,  are  spread  apart  at  the  ends,  thus  presenting 
a  large  surface  to  the  oil.  The  heat  generated  in  the  iron 
and  in  the  coils,  is  readily  communicated  to  the  oil.  The 
heated  fluid  rises,  flows  to  the  top,  and  down  the  sides  of 
the  case,  which  is  deeply  ribbed,  thus  presenting  a  large 
surface  to  the  air.  In  this  way  the  internal  heat  of  the 
transformer  finds  its  way  to  the  external  surface,  and  is 
thence  radiated  into  space. 

A  300  K.W.  transformer  of  this  type,  manufactured  by 
the  Wagner  Electric  Company,  is  shown  in  Fig.  82.  This 
transformer  is  unusually  interesting,  from  the  fact  that 
it  is  wound  for  40,000  volts,  and  is  one  of  a  number  in 
daily  use  in  the  power-house,  and  the  substation  of  the 


STATIC  TRANSFORMERS. 


125 


Telluride   Transmission    Company.     The    transmission   is 
from   Provo  to  Mercur,  Utah,  the  distance  being  nearly 


Fig.  82. 


forty  miles.  The  generator  current  of  700  volts  is  raised 
to  40,000  volts,  and  reduced,  at  the  receiving  end,  to  a 
suitable  voltage  for  supplying  motors  and  lights,  principally 


126       POLYPHASE   APPARATUS   AND   SYSTEMS. 


STATIC  TRANSFORMERS. 


127 


for  mining  operations.  The  essential  features  of  the  self- 
cooled  oil  transformers,  when  designed  for  power  service, 
are  a  liberal  proportioning  of  the  mechanical  parts,  and  a 
corrugated  sheet-iron  case.  The  first  feature  produces  a 
rapid  convection  of  the  internal  heat,  and  facilitates  a  rapid 


Fig.  84. 

oil  circulation.  By  means  of  the  latter  feature,  the  exter- 
nal radiating  surface  is  two  or  three  times  greater  than  that 
which  a  plain  case  would  have. 

Water-Cooled  Oil  Transformers. — When  provided  with 
some  artificial  method  of  cooling  the  oil,  these  transformers 


128   POLYPHASE  APPARATUS  AND  SYSTEMS. 


are  smaller  and  cheaper  to  build  than  those  dependent  for 
cooling  upon  natural  radiation.  There  are  a  number  of 
methods  of  cooling  such  transformers  ;  one  of  these  is  by 
circulating  cold  water  in  a  worm  or  system  of  pipes  sur- 
rounding the  transformer  (Fig.  83) ;  another  method  of 

cooling  is  by  draw- 
ing off  the  oil,  cool- 
ing it,  and  pumping 
it  back,  the  opera- 
tion being  contin- 
u  o  u  s.  A  1,000 
H.P.  oil-cooled 
transformer  is  in 
daily  use  by  the 
Carbide  Manufac- 
turing Company, 
Niagara  Falls.  A 
motor,  pump,  and 
system  of  oil-tanks 
for  circulating  and 
cooling  the  oil  are 
used  to  control  the 
temperature  of  the 
transformer.  The 
oil  is  forced  upward  through  spaces  left  around  and  be- 
tween the  coils,  overflows  at  the  top,  and  passes  down 
over  the  outside  of  the  iron  laminations. 

In  still  another  transformer  the  windings  are  cooled 
by  the  circulation  of  water  in  flat,  thin  ducts,  interposed 
between  the  windings  (Fig.  84).  One  form  of  this  trans- 
former of  low  secondary  voltage  has  flat  copper  tubes  for 
the  secondary  winding,  through  which  cold  water  is  circu- 


Flg.  85. 


STATIC  TRANSFORMERS. 


129 


lated.  The  primary  windings  are  placed  between  the  sec- 
ondaries, so  that  the  water  circulation  in  the  latter  keeps 
both  windings  at  a  low  temperature. 

The  transformer  is  incased  in  a  circular  iron  tank,  with 
solid  base,  and  filled  with  oil.  The  province  of  the  oil 
is  to  cool  the  iron  laminations,  and  also  to  prevent  the 
condensation  of  moisture  from  the  air  on  the  cold  wind- 
ings. 

Another  form  of  oil  transformer  is  cooled  by  means  of 
a  water  jacket,  surrounding  the  case  containing  the  trans- 


Fig.  86. 

former  proper.  Fig.  85  shows  a  Wagner  transformer  of 
this  type.  The  case  is  completely  channelled  by  water 
passages.  A  low  water  pressure  of  10  or  15  pounds  is 
sufficient  for  all  ordinary  requirements  of  service. 

Some  outside  source  of  power  is  required  to  operate  the 
cooling  devices,  which  slightly  reduces  the  total  efficiency 
of  the  transformation.  The  water  coil  may  be  conve- 
niently supplied  by  water  mains,  or,  in  the  case  of  a  water- 
power  transmission,  by  the  water  under  head. 


130       POLYPHASE   APPARATUS   AND    SYSTEMS. 

Air-Blast  Transformers.  —  In  this  transformer  the  cool- 
ing is  effected  by  means  of  a  forced  current  of  air  circulat- 
ing through  the  windings  and  core. 

The  primary  and  secondary  coils  are  separately  wound 
on  formers  and  insulated,  and  then  assembled  in  groups 
(Fig.  86),  the  coils  being  intermingled.  The  groups  are 


Fig.  87. 

assembled  in  the  form  of  a  case,  being  separated  from  one 
another  by  vertical  air  spaces.  The  iron  case  is  then  built 
up  around  the  windings  (Fig.  87),  the  laminations  being 
horizontally  spaced  at  frequent  intervals. 

It    is   evident  that  this  construction   permits   the   most 
complete  ventilation,  as  the  very  heart  of  the  transformer 


STATIC  TRANSFORMERS. 


is  reached  by  the  blast  of  air.  The  flow  of  air  is  con- 
trolled by  means  of  two  dampers,  one  of  which  is  located 
at  the  top  of  the  transformer,  regulating  the  air  between 
the  windings;  the  other  is  on  the  side  of  the  frame,  and 
controls  the  flow  of  air  through  the  core.  Fig.  88  shows 
the  arrangement  of  iron  and  copper  parts  and  ventilating 
ducts ;  and  Fig.  89  a  completed  transformer  in  its  frame. 


Fig.  88. 

The  apparatus  for  funishing  the  air  blast  consists  of  a 
blower,  and  is  usually  operated  by  a  motor,  the  air  being 
delivered  to  the  transformer  by  means  of  a  flue.  The 
volume  of  air  required  for  cooling  purposes  varies  with  the 
number,  size,  and  efficiency  of  the  transformers. 

The  following  table  gives  the  volume  and  pressure  of  air 
required  for  transformers  of  various  sizes,  of  the  average 
efficiency. 


132   POLYPHASE  APPARATUS  AND  SYSTEMS. 


°1 

U.     K 
O     W 

K 

& 

«  S  g 

w 

J      <" 

S  1  « 

*ll 

%  EJ 

w    ^ 

1$ 

%  S 

H£'| 

u   u 

.    O    g 

fe      £      V3 

fi   w 

H    ^ 

£   •<  ^ 

^M 

W  « 

0.     ^ 

§    w  ^ 

D     O'    < 

PH  W 

H  H 

ts>i  ^ 

s  0 
O 

O 

U£H 

W 

300 

5° 

40" 

375 

1,  800 

•30 

250 

.25 

900 

IOO 

50" 

350 

3,200 

.40 

35o 

.60 

1,  800 

200 

60" 

325 

5,9°° 

•50 

600 

I.IO 

2,700 

300 

70" 

310 

8,800 

.60 

850 

2.25 

4,5°° 

500 

80" 

310 

13,000 

.80 

1,300 

4-25 

6,750 

750 

90" 

295 

17,600 

.90 

i,  800 

6-75 

7,500 

1,250 

IOO" 

280 

23,600 

I. 

3,000 

12. 

From  the  table  it  will  be  seen  that  the  power  consumed 
in  cooling  the  transformers  is  less  than  .  I  of  i  per  cent  of 
the  output  of  the  transformers.  If  the  transformers  have 
an  efficiency  of  97.5  per  cent  at  full  load,  the  total  effici- 
ency of  transformation  is  reduced  to  97.4  per  cent  by  the 
use  of  the  air  blast  —  a  perfectly  negligible  quantity. 

In  case  of  damage  to  the  cooling  arrangement,  the  trans- 
formers can  operate  for  a  few  hours  without  the  air  blast. 
It  is  desirable,  however,  to  always  provide  this  apparatus  in 
duplicate. 

The  material  protecting  the  primary  and  second  coils 
and  the  windings  from  the  case,  has  for  the  same  thickness 
a  considerably  greater  insulating  property  than  oil  or  air. 

Operation  of  Air-Blast  Transformers When  transform- 
ers of  this  type  are  run  in  groups  or  "banked"  together, 
care  should  be  take  that  the  air  enters  each  transformer 
at  the  same  pressure,  otherwise  the  transformers  will  heat 
unequally.  This  can  be  accomplished  by  having  the  flue 
from  the  blower  to  the  transformer  of  such  area  that  the 
velocity  of  the  air  will  not  exceed  200  feet  per  minute. 


STATIC  TRANSFORMERS. 


133 


The  most  desirable  installation  of  the  transformer  is  over 
a  closed  chamber  of  sufficient  size  to  admit  inspection  of 
the  windings.  Unequal  pressure  in  different  transformers 


Fig.  89. 

may  be  compensated  for  by  means  of  the  two  dampers 
with  which  each  transformer  is  provided.  The  tempera- 
ture of  the  outgoing  air  affords  a  ready  means  of  deter- 
mining the  proper  amount  of  air  to  be  admitted  to  each 


134   POLYPHASE  APPARATUS  AND  SYSTEMS. 

transformer.  The  air  supply  is  sufficient,  if,  at  full  load, 
it  does  not  heat  more  than  20°  Centigrade  above  the  sur- 
rounding atmosphere. 

Transformers  of  different  capacities,  or  even  of  the 
same  capacity,  should  not  be  operated  in  parallel  unless 
they  have  the  same  electrical  constants  ;  otherwise,  the 
load  will  be  unequally  divided.  Parallel  connections  should 


Converter 


Fig.  90. 

have  the  least  possible  resistance  for  the  same  reason. 
Fig.  90  shows  the  installation  and  connections  of  air-blast 
transformers  in  a  long-distance  power  transmission. 

Natural-Draft  Transformers.  —  Transformers  of  this  type 
are  self-cooled  by  a  natural  circulation  of  air.  Fig.  91 
shows  a  transformer  without  its  case.  They  are  designed 
to  have  very  large  radiating  surfaces,  compared  with  their 
capacity.  As  usually  constructed,  the  windings  are  on  the 


STATIC   TRANSFORMERS. 


135 


outside  of  the  core,  instead  of  being  surrounded  by  it. 
Every  facility  is,  therefore,  present  for  the  radiation  of  heat 
from  the  coils.  The  transformer  is  mounted  upon  a  solid 
foundation,  and  then  covered  with  a  corrugated  sheet-iron 
cylinder,  provided  with  bottom  openings  and  a  ventilating 
roof.  This  construction  allows  a  free  and  natural  circula- 
tion of  air  through  the  casing  and  around  the  transformer. 
These  transformers  are 
built  for  10,000  volts 
or  15,000  volts,  and  of 
capacities  from  5  to  50 
K.W.  They  are,  as 
might  be  expected, 
more  expensive  than 
either  oil  or  air-blast 
transformers.  They 
have  the  compensating 
advantages  of  not  re- 
quiring any  artificial 
cooling  device. 

Efficiency  and  Loss- 
es.—  The  character- 
istic efficiency  curve  of 
a  well-designed  trans- 
former shows  a  high 
efficiency  at  all  but 
very  light  loads.  In 
Fig.  92  the  efficiency 
of  a  250  K.  W.,  60  Fig.  91. 

cycle  transformer  does 

not  fall  below  90  per  cent  from  £  load  to  about  T^  load. 
Good    efficiency,    at    light    loads,    is   a    valuable    feature, 


136       POLYPHASE   APPARATUS   AND    SYSTEMS. 

epecially  in  motor  and  lighting  transformers,  where  the 
average  load  rarely  makes  a  demand  of  more  than  one- 
half  of  the  transformer  capacity.  The  efficiencies  of  the 
250  K.W.,  taken  from  the  curve,  are  as  follows  : 

TJQ  load 87      per  cent 

£  load 94.6  per  cent 

-J-  load 97      per  cent 

f  load 97.7   per  cent 

full  load 98      per  cent 

i^-  load 98.1   per  cent 


JLUU 

^ 

IX 

*<^—  ** 

, 

7 

1 

/ 

'C:    80 
Ul 

0 

cS    '° 

4 

an 

en 

>io             %>                   %                      H                      Full 
Per  Cent,  of  Load 
Fig.  92. 

The  losses  in  a  transformer  consist  only  of  copper  and 
iron  losses.  The  former  vary  with  the  load,  while  the  iron 
or  core  losses  remain  about  the  same  for  all  loads.  It  is 
necessary,  therefore,  in  order  to  obtain  good  efficiency  at 
light  loads,  to  reduce  the  core  losses  to  a  minimum. 
Judging  from  the  shape  of  the  curve  in  Fig.  92,  the  core 


STATIC  TRANSFORMERS. 


137 


loss  must  be  small.  This  is  shown  to  be  the  case  in  Fig. 
93,  which  gives  the  watts  lost  in  the  iron  of  the  same 
transformer,  and  also  the  corresponding  exciting  current. 
At  full  voltage,  the  exciting  current  is  2.3  amperes,  and  the 
core  loss  3,380  watts,  or  i£  per  cent  of  the  full-load  input 
of  the  transformer.  The  exciting  current  being  a  lagging 
current  does  not,  of  course,  represent  a  corresponding 
waste  of  energy.  The  loss  in  the  copper  conductors  is 
only  |  of  i  per  cent.  By  reducing  the  amount  of  copper 
in  both  primary  and  secondary  coil,  say,  one-half,  we  obtain  a 
proportionately  increased 
loss  in  the  copper,  or  a 


reduction  in  the  efficiency    2300 
of  transformer   from   98 
per  cent  to  approximately 
97.5   per  cent.     But  the   lxo 
total    cost   of   the  trans-    m 
former     is    thereby    de-    m 
creased    from    10   to   20      °; 
per  cent.                                    ] 
It  is  not  always  wise  to 
select  the  more  efficient 

/ 

^ 

/ 

<& 

^ 

fa 

** 

^ 

£ 

s 

Sf 

at 

s 

A 

s 

'v 

^ 

/ 

I 

/ 

o< 

/ 

/ 

/ 

/ 

s 

s 

^. 

^ 

>       .4 

i     i 

j 

\       1.2     1.6     2.0    2.4 

2.8     3.2    3.< 
i         ill 

>      400    800   1200  1600  2000  2400  2800  3200  36C 

Pig.  93. 

transformer,  especially  in  water-power  transmissions,  where 
the  chief  item  in  the  cost  of  delivered  power  is  the  interest 
on  the  plant,  nor  in  plants  where  there  is  not  a  demand  for 
every  horse-power  developed.  As  an  illustration,  take  the 
case  of  a  power  transmission  of  1,000  H.P.  using  the 
cheaper  transformer,  which  has  an  efficiency  of  97.5  per 
cent.  The  power  delivered  is  about  I  per  cent,  or  10  H.P., 
less  than  with  the  more  efficient  step-up  and  -down  trans- 
formers. If  a  market  were  found  for  every  horse-power 
transmitted  at,  say  $30  per  H.P.  per  year,  the  loss  in  reve- 


138       POLYPHASE  APPARATUS  AND   SYSTEMS. 

nue  to  the  power  company  would  be  $300  a  year.  As  a 
partial  offset,  there  would  be  the  interest  on  the  difference 
in  the  first  cost  of  the  transformers.  Few  water-power 
transmissions,  however,  are  run  at  their  full  capacity. 
When  such  is  the  case,  the  power  company  is  usually  war- 
ranted in  buying  the  expensive  transformer.  In  the  trans- 
mission of  steam-generated  power,  fuel  is  generally  the 
most  important  single  factor  in  the  make-up  of  the  total 


024  6  8  10 

AMPERE  PRIMARY 

Figr.  94. 

cost  of  power,  and,  as  a  rule,  the  most  efficient  transform- 
ing devices  should  be  used. 

Regulation.  —  Regulation  in  a  transformer  is  the  per- 
centage drop  of  secondary  voltage  from  no  load  to  full  load, 
the  primary  voltage  remaining  constant.  Good  regulation 
is  more  desirable  in  a  transformer  than  in  a  generator,  as 
there  is  no  means  in  the  former  apparatus  of  compounding 
for  the  voltage  drop.  On  a  non-inductive  load,  the  regula- 
tion is  equal  to  the  I.R.  drop  of  the  secondary.  On  an 
inductive  load,  the  regulation  is  the  drop  due  to  the  result- 


STATIC  TRANSFORMERS.  139 

ant  of  the  ohmic  and  inductive  components  of  resistance. 
The  regulation  in  this  case  is  the  same  as  the  impedance. 
Fig.  94  shows  the  I.R.  and  the  impedance  drop  of  a  250 
K.W.,  6ocycle  transformer.  The  impedance  curve  is 
obtained  by  short-circuiting  the  secondary,  which  gives  the 
most  inductive  condition  of  operation,  and  measuring  the 
voltage  drop  from  no  load  to  full  load.  The  non-inductive 
regulation  is  seen  to  be  .7  of  i  per  cent,  and  regulation  on 
full  inductive  load  4.29  per  cent. 


140       POLYPHASE   APPARATUS   AND   SYSTEMS. 


CHAPTER    VIII. 

STATION    EQUIPMENT   AND   GENERAL 
APPARATUS. 

Switchboard  for  Generating  Apparatus.  —  The  station 
equipment  of  a  polyphase  plant  is  practically  the  same  as 
that  of  a  simple  single-phase  installation.  The  chief  dif- 
ference lies  in  the  new  features  necessitated  by  the  use  of 
large  generating  units. 

In  a  power  plant,  consisting  of  a  number  of  generators, 
the  modern  practice  is  to  instal  one  panel  for  each  genera- 
tor and  one  panel  for  the  exciter  dynamos.  The  generator 
panel  will  contain  the  generator  switches,  current  and 
potential  measuring  instruments,  fuses,  field  switch,  and 
rheostats.  The  exciter  panel  usually  has  mounted  on  it 
the  exciter  switches,  main  line  switches,  and  measuring  and 
controlling  instruments  for  the  total  output  of  the  generators. 

Marble  is  the  material  on  which  the  instruments  are 
mounted.  Slate  is  not  suitable  for  pressures  greater  than 
600  volts,  on  account  of  its  liability  to  current  leakage, 
due  to  the  presence  of  metallic  veins.  The  instruments 
are  mounted  on  the  face  of  the  panel,  the  electrical  con- 
nections for  the  appliances  being  made  behind  the  panel. 
Separate  panels  are  connected  electrically  by  copper  tie- 
bars,  uniting  the  ous-bars  at  the  back.  They  are  con- 
nected mechanically  by  bolts  through  the  angle-iron  frame 
behind  the  board. 


I 
STATION   EQUIPMENT  AND   APPARATUS.       141 

Panels  for  single  units  are  sometimes  made  up  to  suit 
requirements  by  uniting,  piecemeal,  smaller  marble  panels 
containing  the  various  appliances.  The  angle-iron  frame- 
work, used  to  support  the  panels,  is  arranged  to  form  part 
of  each  individual  panel.  Fig.  95  shows  one  of  the  stand- 
ard forms  of  three-phase  generator  panels,  composed  of 
two  pieces  only.  These  panels  usually  consist  of  a  slab  of 
marble  about  62"  X  30"  X  2",  with  a  sub-base  28"  X  30" 
x  2" ',  supported  by  a  metal  frame.  The  panel  will  ordi- 
narily contain  the  following  instruments : 

3  fuse-holders,  or  6  for  two  circuits, 

2  pilot  lamps,  one  for  generator  and  one  for  exciter, 
i  current  indicator,  or  ammeter,  for  generator, 

i  potential  indicator,  or  voltmeter,  for  generator, 
i  ground  detector,  plug,  and  receptacle, 
i  main  three-pole  switch  for  three-phase  generator, 
i  double-pole  switch  and  fuse-holder,  for  exciter, 
i  rheostat  for  generator  field, 
i  rheostat  for  exciter  field, 
And  behind  the  panel, 

3  lightning  arresters, 
i  station  transformer, 
i  ground  detector. 

Fig.  96  indicates  diagrammatically  the  apparatus  and 
connections  of  the  panel  just  described. 

In  the  selection  of  instruments  for  a  switchboard,  a  lib- 
eral excess  allowance  should  be  made.  For  instance,  the 
range  of  the  ammeters  should  exceed  the  nominal  rating 
of  the  generators  at  least  25  per  cent.  The  voltmeters 
should  have  the  same  conservative  rating.  All  switches, 
connections,  bus-bars,  and  terminals  should  be  designed  to 
carry  full-load  current  continuously,  without  appreciable 
heating. 


142        POLYPHASE   APPARATUS   AND    SYSTEMS. 


For  the  measurement,  regulation,  and  control  of  the 
output  of  generators  on  a  large  scale,  as  at  Niagara,  ordi- 
nary methods  and  experi- 
ence do  not  apply.  The 
maximum  current  in  any 
part  of  the  bus-bars  in  the 
plant  at  Niagara  is  about 
3,000  amperes.  The  dif- 
ficulties met  with  here 
were  the  necessity  of  in- 
sulating the  bus-bars,  thus 
compelling  the  use  of 
round  bars ;  the  virtual 
resistance  or  skin  effect, 
which  reduced  the  effec- 
tive copper  area,  and  the 
small  radiating  surface  of 
round  bars,  as  compared 
with  flat  strips.  These 
difficulties  were  overcome 
by  the  use  of  hollow  cop- 
per rods. 

The    design  of    the 
switches    for    the     same 
installation,    capable  of 
opening,    without    dam- 
age,   circuits    conveying 
5,000     H.  P.    at    2,000 
volts,    was    a    difficult 

Figr-  95'  problem.       The    main 

switches  used  for  this  purpose  are  pneumatically  operated. 

The  increasing  use  of  initial  high-voltage  generators  has 


STATION    EQUIPMENT  AND   APPARATUS.       143 


144   POLYPHASE  APPARATUS  AND  SYSTEMS. 

necessitated  the  development  of  special  switches.  While 
other  and  slower  methods  of  opening  a  high-voltage  circuit 
of  great  current  volume  are  in  daily  use,  the  necessity  may 
arise  for  suddenly  interrupting  the  main  circuit,  and  the 
proper  appliances  should  be  found  in  every  installation  of 
this  character. 

Switchboards  for  Power-Transmission  Apparatus —  The 
switchboard  equipment  of  the  power  house  of  a  trans- 
mission plant  includes  panels  for  the  step-up  transformers, 
when  such  apparatus  is  used,  and  a  high  potential  panel 
board,  which  serves  as  a  connecting  link  between  the 
transmission  lines  and  the  station  wiring. 

Fig.  97  shows  the  connections,  in  the  generating  station 
of  a  large  power  transmission  plant,  consisting  of  four 
1,000  H.P.  generator  units,  one  of  which  is  held  as  a 
spare.  The  three-phase  machines  are  wound  for  800  volts, 
and  are  each  connected  to  the  generator  panels  by  cables 
of  550,000  circular  mils.  Behind  the  panels  are  two  sets 
of  bus-bars,  which  are  connected  to  the  measuring  and  con- 
trolling instrument  on  the  central  or  exciter  panel.  From 
the  exciter  panel  the  total  output  of  the  generator  is  led 
by  two  circuits  of  6  wires,  each  conductor  consisting  of 
two  800,000  C.M.  cables,  to  the  step-up  transformer  board, 
where  any  combination  of  generators  and  transformers  can 
be  made  by  means  of  the  double-throw  switches.  The 
high  potential  board  contains  the  switches  for  making  any 
desired  connection  of  lines  and  step-up  transformers.  The 
transmission  system  is  seen  to  consist  of  two  pole  lines, 
each  composed  of  two  circuits  of  three  wires.  By  means 
of  the  distributing  board,  the,  total  output  of  the  station 
can  be  transmitted  over  either  pole  line,  or,  in  parallel, 
over  both  lines.  The  output  can  also  be  divided  among 


STATION   EQUIPMENT   AND   APPARATUS.       145 


Three-Phaat 

Generators 


146   POLYPHASE  APPARATUS  AND  SYSTEMS. 

the  two  transmission  lines.  One  of  the  generators  can 
deliver  power  to  one  of  the  pole  lines,  and  the  remaining 
two  active  generators  run  in  parallel  over  the  other  pole 
line,  or,  singly,  over  separate  transmission  lines. 

A  simple  arrangement  of  transformer  panels  for  smaller 
plants  is  illustrated  in  Fig.  98.  The  generating  plant  is 
composed  of  two  units.  The  lower  row  of  double-throw 
switches  connects  the  low-tension  primaries  of  the  trans- 
formers to  either  set  of  bus-bars  on  the  generator  board. 
The  upper  row  of  switches  is  used  for  connecting  the  high- 
tension  leads  from  the  transformer  secondaries  to  either 
one  or  both  of  the  transmission  lines.  The  small  panel  on 
the  right  contains  the  switch  for  connecting  the  transmis- 
sion lines  in  parallel. 

The  connections  and  instruments,  in  the  generating  sta- 
tion and  substation,  of  a  two-unit  transmission  plant,  are 
diagrammatically  shown  in  Fig.  99.  The  switchboard  equip- 
ment in  the  power  station  is  composed  of  two  generator 
panels,  one  exciter  panel,  and  two  transformer  panels. 
The  generator  panels  are  provided  with  but  one  set  of 
bus-bars.  At  the  substation  are  shown  two  step-down 
transformer  panels.  The  additional  panels,  to  be  installed 
will  be  determined  by  the  extent  and  character  of  the  cur- 
rent-consuming appliances.  Provision  for  an  increase  in 
the  plant  can  be  made  without  disturbing  the  general 
arrangement  here  shown,  by  simply  adding  the  necessary 
generator  and  transformer  panels. 

Lightning  Protection There  is  no  problem  with  which 

the  electrical  engineer  has  to  deal,  that  presents  greater 
difficulties  in  the  way  of  a  positive  solution,  than  that  of 
lightning  protection.  The  uncertainty  among  the  highest 
authorities  as  to  the  exact  nature  of  lightning  phenomena 


STATION   EQUIPMENT  AND   APPARATUS.       147 


is  largely  accountable  for  this  state  of  affairs.     The  oscilla- 
tory character  of  a  direct  lightning  stroke  has  been  estab- 


High  Tenson  Switches 
Connecting  Secondaries 
of  Step-up  Transformers 
to  Lines 


\     I 

Switches  Connecting  Primaries 
of  Step-up  Transformer* 
to  Bus  Bars 


Switches  Connecting 
Circuits  in  Parallel 


Fig.  98. 


lished  beyond  a  doubt,  but  experience  with  lightning  effects 
would  indicate  that  some  of  the  disruptive  discharges  act 
mainly  in  one  direction.  For  this  reason  no  one  single 


148   POLYPHASE  APPARATUS  AND  SYSTEMS. 


TTYFIM  Rheottat  |'T  I 

*~YD.F.S.T.  Switch  __  _3j 

"     I 


Feeder 


Feeder 


Fig.  99. 


STATION   EQUIPMENT  AND   APPARATUS.       149 

device  can  be  depended  on  to  protect  electrical  apparatus 
from  all  kinds  of  lightning  phenomena.  In  other  words, 
there  is  not,  and  cannot  be,  a  universal  lightning  arrester. 
The  discharge  current  from  any  system  of  conductors,  pro- 
duced by  the  various  phenomena,  can  be  described  under 
three  general  headings : 

ist.  —  The  direct  discharge  due  to  the  transmission  lines 
being  in  the  direct  path  of  the  lightning  stroke. 

2d.  —  The  cumulative  discharge,  due  to  a  gradual  and 
sometimes  enormous  rise  of  potential  from  a  changing 
electrostatic  condition  of  the  atmosphere. 

3d.  —  The  secondary  discharge  due  to  secondary  cur- 
rents induced  in  the  lines  by  parallel  lightning  strokes. 

There  are  other  kinds  of  lightning  discharges  from 
transmission  lines,  which  partake,  more  or  less,  of  the 
character  of  the  above,  but  do  not  differ  greatly  in  their 
effects. 

Provision  for  the  protection  of  the  station  apparatus 
should  be  made,  not  only  in  the  station  itself,  but  along 
the  transmission  lines  as  well.  The  means  usually  em- 
ployed for  protecting  the  lines,  consist  of  guard  wires  out- 
side of  the  conductors,  or  even  of  one  guard  wire  strung  at 
the  top  of  the  pole.  It  is  better  to  ground  this  wire  at 
every  fourth  or  fifth  pole.  In  long-distance  transmissions, 
it  is  also  well  to  install,  every  ten  miles  or  so,  line  arrest- 
ers, similar  to  those  used  in  the  station.  The  guard  wires 
protect  the  conductors  from  the  direct  lightning  stroke,  by 
discharge  to  the  ground.  They  also  have  a  dampening 
effect  on  the  secondary  induced  currents,  and  those  due  to 
a  change  of  the  electrostatic  equilibrium. 

Commercial  Lightning  Arresters From  the  foregoing, 

it  will  be  understood  that  there  is  a  good  deal  yet  to  be 


150       POLYPHASE   APPARATUS   AND    SYSTEMS. 

learned  about  the  most  suitable  form  of  lightning  protec- 
tion for  alternating  current  apparatus.  Nevertheless,  ex- 
perience has  narrowed  down  the  many  ancient  devices  to 
one  type  of  arrester,  i.e.,  an  arrester  composed  of  a  number 
of  metal  balls  or  cylinders,  separated  by  short  air  gaps. 
Fig.  100  shows  one  of  this  class,  devised  by  Mr.  Wurts  of 


Fig,  1OO. 

the  Westinghouse  Company.  It  is  seen  to  consist  of  seven 
cylinders,  each  one  inch  in  diameter  and  three  inches  long, 
and  separated  by  spaces  ^  of  an  inch.  The  particular 
arrester  shown  is  of  the  double-pole  type,  and  designed  for 
alternating  circuits  of  1,000  volts.  When  the  discharge 
takes  place  simultaneously  from  two  separate  conductors,  a 
short  circuit  would  follow,  if  the  arc  were  not  immediately 
interrupted.  It  is  found,  with  the  arrester  described,  that 


STATION   EQUIPMENT  AND   APPARATUS.       151 


the  flash  is  instantaneous,  and  is  not  followed  by  an  arc. 
The  passage  of  the  static  discharge  through  the  arrester  is 
evidenced  by  burns  or  pit-marks  on  the  cylinder  surfaces. 
These  can  be  rotated 
on  their  axes,  in  order 
to  bring  fresh  surfaces 
opposite  each  other. 

Another  form  of 
this  lightning  arrester 
is  shown  in  Fig  101. 
This  device,  made  by 
the  General  Electric 
Company,  consists  of 
a  combination  of 
short  metal  cylinders 
and  a  graphite  resist- 
ance. The  single-pole 
arrester  for  1,000 
volts  has  one  spark 
gap  of  ^V  of  an  inch, 
separating  two  metal 
cylinders  two  inches 
in  diameter  and  two 
inches  long.  A  non-inductive  graphite  resistance  is 
placed  in  series  with  the  ground  wire.  The  2,000  volt 
single-pole  arrester  has  three  cylinders  and  two  air  gaps 
of  approximately  ^V  of  an  inch  each,  and  a  graphite  re- 
sistance. 

The  arrester  first  described  is  made  of  an  alloy  of  zinc 
and  antimony,  and  will  operate  with  better  results  than 
when  the  cylinders  are  made  of  copper.  The  last-de- 
scribed arresters  have  bronze  cylinders.  The  arc-extin- 


Fig.  101. 


152   POLYPHASE  APPARATUS  AND  SYSTEMS. 

guishing  action  of  these  arresters  is  dependent  mainly 
upon  the  cooling  effect  of  large  metal  masses,  and  not 
materially  upon  the  kind  of  metal.  This  cooling  effect  is 
increased  by  the  introduction  of  the  non-inductive  resist- 
ance, which,  even  in  the  event  of  the  formation  of  an  arc 
of  short  circuit,  would  materially  limit  the  volume  of  cur- 
rent. The  reversal  of  the  alternating  current  itself  extin- 
guishes the  arc,  in  the  absence  of  vapor,  which  cannot 
arise  from  the  chilled  metal  surfaces.  This  is  proved  by 
the  fact  that  a  lightning  arrester,  which  will  not  short- 
circuit  on  2,400  volts  alternating,  will  hold  an  arc  on 
500  volts  direct-current. 

Installing  Lightning  Arresters. — The  principle  on  which 
a  lightning  arrester  is  selected  for  any  particular  voltage 
is,  that  it  must  be  the  weakest  spot  in  the  line.  The  volt- 
age required  to  jump  all  the  air  spaces  should  be  less  than 
that  which  will  puncture  the  insulation  of  the  apparatus  to 
be  protected.  The  proper  number  of  gaps  for  different 
voltages  can  be  determined  by  experiment  with  plants  in 
actual  operation  It  has  been  ascertained  by  tests  at 
Niagara  that,  for  11,000  volts,  14  air  gaps  of  ^  inch  in 
connection  with  carbon  resistances,  afford  full  protection 
with  a  margin  of  safety.  Circuits  above  2,000  volts  are 
protected  by  standard  2,000  volt  arresters  placed  in 
series. 

Fig.  1 02  shows  the  connections  and  method  of  installing 
the  G.E.  arrester  for  10,000  volt  circuits. 

The    oscillatory    character    of   the    lightning    discharge 

gives  rise  to  great  self-induction  in  the  circuit.     It  would 

seem  as  if  a  choking  coil  placed  between  the  arrester  and 

"the  electrical  apparatus  would  offer  such  resistance  to  the 

discharge  as  to  force  it,  under  all  conditions,  through  the 


STATION  EQUIPMENT  AND  APPARATUS.      153 

arrester,  and  thence  to  the  ground.  In  actual  service  it 
has  been  found  that  the  choking  coil  does  not  always  offer 
this  resistance,  and  for  this  reason  its  usefulness  has  been 
questioned. 

The  uncertainty  of  action  of  one  choking  coil  in  the  cir- 
cuit is  no  proof  of  its  inefficiency.     There  is  good  reason 


Ground 


Fig.  102. 

to  believe  that  this  oscillatory  discharge  has  a  wave-like 
motion,  with  maxima  and  minima  points.  If  the  arrester 
happens  to  be  placed  at  a  point  of  interference,  —  a  nodal 
point  —  the  coil  cannot  force  the  discharge  through  the 
arrester.  This  difficulty  may  be  overcome  by  the  use 
of  a  series  of  coils  and  arresters,  arranged  as  illustrated 
in  Fig.  103.  This  shows  one  end  of  a  2,000  volt, 
three-phase  system  of  conductors.  The  choking  coil 
may  be  made  by  winding  150  feet  of  the  line  wire  into 


154       POLYPHASE  APPARATUS  AND   SYSTEMS. 

a  coil,  the   inside   diameter   of  which  is  not  less  than   1 5 
inches. 

The  grounding  of  lightning  arresters  must  be  most 
carefully  made,  as  upon  attention  to  this  depends  the  reli- 
ability of  the  working  of  the  arresters.  The  connections 
to  ground  and  line  should  be  made  by  short  straight  wires, 
of  not  less  than  No.  4  size.  A  metal  plate,  or  long  pipe, 


Fig.  1O3. 

should,  serve  as  the  ground  terminal,  embedded  in  coke  or 
sunk  in  damp  ground  if  possible.  Fig.  104  illustrates  a 
very  effective  method  of  grounding  a  line  arrester.  It  is 
better  to  use  a  ground  plate  with  station  arresters. 

Synchronizing  Devices. —  The  ordinary  method  of  deter- 
mining whether  two  alternating  generators  are  in  parallel, 
is  by  the  use  of  two  transformers  and  lamps,  as  described 
in  Chapter  III.  This  is  an  excellent  and  effective  method, 


STATION   EQUIPMENT  AND   APPARATUS.       155 

and  is  reliable   under   almost    all    conditions.     A    special 
device,  called  the  acoustic  synchronizer,  is  sometimes  used. 


Fig.  1O4. 


This  consists  of  two  electro-magnets,  actuating  two  enclosed 
diaphragms  by  currents  from  the  machines  to  be  synchro- 


156       POLYPHASE  APPARATUS   AND   SYSTEMS. 

nized.  When  the  generators  are  out  of  phase,  the  instru- 
ment gives  out  a  loud  pulsating  note,  which  grows  feebler 
as  synchronism  is  approached.  The  acoustic  synchronizer, 
while  accurate,  is  not  vigorous  in  its  action,  especially  in 
noisy  stations  and  on  circuits  of  low  frequency.  An  instru- 
ment called  the  synchronoscope  has  been  developed  for  low- 
frequency  circuits,  by  the  Westinghouse  Company.  This 
apparatus  is  similar  in  appearance  to  a  round-dial  voltmeter 
or  ammeter.  When  no  current  flows  between  the  ma- 
chines, the  needle  stands  at  zero.  When  there  is  a  phase 
difference,  a  slight  current  flows  around  a  magnet,  which 
deflects  the  needle. 

Insulators. —  On  transmission  lines,  conveying  currents 
at  potentials  of  10,000  volts,  or  thereabouts,  and  over,  it  is 
common  practice  to  employ  porcelain  insulators.  Glass  is 
generally  used,  and  has  been  found  most  satisfactory  as  an 
insulating  line  material  for  potentials  lower  than  10,000 
volts. 

Line  insulators  for  heavy  service,  such  as  high-tension 
transmission  of  power,  should,  in  an  eminent  degree,  pos- 
sess two  qualities : 

i  st. — Thorough  insulation  under  all  conditions  of  operation. 
2d.  —  Great  mechanical  strength. 

When  formed  into  large  masses,  porcelain  is  supposed 
to  be  superior  to  glass  in  both  these  respects.  Glass  is 
an  almost  absolute  non-conductor  of  electricity,  but  is  said 
to  be  hygroscopic,  i.  e.,  condenses  water  on  its  surface  from 
the  atmosphere,  and  thus  allows  a  leakage  of  the  current. 
When  massive,  glass  is  somewhat  difficult  to  anneal,  and 
hence  is  not  always  as  strong  mechanically  as  desirable. 


STATION    EQUIPMENT   AND   APPARATUS.       157 


Porcelain  for  insula- 
tors should  be  thorough- 
ly vitrified  and  homo- 
geneous. The  material 
should  be  absolutely 
non-absorbent  of  mois- 
ture, and  sufficient  to 
insulate  the  line  even 
without  the  surface 
glazing.  Poor  porcelain 
can  easily  be  detected 
by  the  appearance  of  the 
fracture,  and  its  porous 
quality  by  soaking  in  red 
ink.  Well-vitrified  por- 
celain will  show  no  signs 
of  ink  when  washed ; 
the  poor  material  will 
readily  absorb  it.  An 
inch  thickness  of  porous 
porcelain  will  be  punc- 
tured by  10,000  volts, 
while  the  same  thickness 
of  vitrified  material  has 
failed  to  break  down  un- 
der a  pressure  of  over 
100,000  volts.  Only  the 
general  character  of  por- 
celain insulators  can  be 
determined  in  this  rough 
manner.  To  determine 
the  actual  insulating 


158       POLYPHASE   APPARATUS   AND    SYSTEMS. 

strength,  each  insulator  should  be  submitted  to  a  high 
potential  test. 

This  test  is  best  made  by  placing  a  number  of  insulators 
inverted  in  a  metal  pan,  filled  with  a  brine  solution  to  the 
depth  of  two  inches.  The  brine  also  fills  the  pin  holes.  In 
each  pin  hole  is  placed  a  metal  rod.  All  the  rods  are  con- 
nected to  one  terminal  of  a  high-potential  circuit,  and  the 
pan  to  the  other.  The  testing  pressure  used  is  generally 
about  40,000  volts  for  25,000  volts  service.  When  the 
circuit  is  closed,  the  defective  insulators  are  punctured,  and 
are  manifested  by  a  shower  of  bright  sparks.  Fig.  105 
shows  a  group  of  high-tension  porcelain  insulators. 

While  porcelain  is  a  superior  material  for  insulators,  its 
much  greater  cost  than  glass  is  a  serious  drawback.  Ex- 
perience in  their  manufacture  has  largely  overcome  the 
difficulty  of  properly  annealing  large  glass  insulators.  In 
dry  climates,  the  hygroscopic  property  of  glass  is  practi- 
cally nil. 

Under  such  conditions  it  would  seem  as  if  glass  insu- 
lators would  be  entirely  satisfactory,  even  for  the  highest 
voltages  that  may  be  commercially  employed.  It  is  not  at 
all  improbable  that,  with  the  experience  to  be  obtained  as 
their  use  increases,  glass  insulators  will  replace  porcelain 
insulators.  They  have  been  already  successfully  used  for 
very  high  voltages, — notably  in  the  Provo  transmission, 
which  employs  40,000  volts,  this  being  the  highest  voltage 
yet  attempted  in  practice.  The  insulator  known  as  the 
Provo  type  is  illustrated  in  Fig.  106.  It  is  a  triple  petti- 
coat insulator,  having  a  diameter  of  7  inches  and  a  height 
of  6  inches.  It  weighs  4  Ibs.  7  oz. 

A  novel  insulator,  embodying  the  insulating  properties 
of  glass  and  the  non-hygroscopic  property  of  porcelain,  has 


STATION   EQUIPMENT  AND  APPARATUS.       159 


Fig.  1O6. 


160   POLYPHASE  APPARATUS  AND  SYSTEMS. 

recently  been  brought  out.  This  insulator  consists  of  a 
porcelain  body  and  an  inner  glass  sheath,  containing  the 
screw  pin  hole. 

Pressure  Regulators.  —  One  form  of  regulator,  for  vary- 
ing the  pressure  of  an  alternating-current  circuit,  may  be 
likened  to  an  induction  motor  with  its  armature  blocked 
so  as  to  remain  stationary,  but  which  at  the  same  time  is 
capable  of  being  placed  in  various  positions,  thereby  chan- 
ging the  mutual  induction  of  the  coils.  As  ordinarily  con- 
structed, the  regulator  consists  of  what  corresponds  to  the 
field  and  to  the  armature  of  an  induction  motor.  A  hol- 
low cylindrical  structure  built  of  laminated  iron  is  provided, 
on  its  interior  surface,  with  four  slots,  in  which  are  placed 
two  coils  at  right  angles  to  each  other.  Inside  of  these 
coils  a  movable  laminated  core  is  placed,  in  such  a  manner 
that  its  position  can  be  changed  with  respect  to  the  field. 
The  winding  of  the  primary  or  field  is  connected  across 
the  lines.  While  in  the  induction  motor,  the  armature,  or 
secondary  winding,  is  short-circuited  upon  itself,  in  the  reg- 
ulator the  armature  or  secondary  winding  is  connected  in 
series  with  the  circuit  so  as  to  add  its  voltage  to,  or  sub- 
tract it  from,  that  of  the  line,  according  to  its  relative 
position  in  regard  to  the  primary  winding.  Since  the  reg- 
ulator has  some  self-induction,  and  requires  magnetizing 
current,  the  maximum  possible  boosting  obtainable  is  about 
10  per  cent  less  than  the  minimum  reducing  effect.  The 
arrangement  and  the  connections  of  this  regulator  can  be 
seen  in  diagram  Fig.  107. 

Another  type  of  regulator,  built  on  the  same  lines,  is 
sometimes  constructed  to  take  care  of  all  the  branches  of 
a  two-  or  three-phase  circuit.  The  regulator  described 
enables  the  voltage  of  a  circuit  to  be  raised  and  lowered 


STATION   EQUIPMENT  AND   APPARATUS.       l6l 

without  change  of  connections,  and  adjustments  of  pressure 
are  obtained  by  imperceptible  degrees. 

The  Stillwell  regulator  is  another  form  of  apparatus  for 
raising  or  lowering  the  pressure  in  feeder  wires.  It  is  a 
transformer,  the  primary  of  which  is  connected  across  the 


Fig.  107. 


circuit  and  the  secondary  of  which  is  in  series  with  the 
feeder  whose  voltage  is  to  be  regulated.  The  cut  (Fig.  108) 
shows  the  general  appearance  of  this  regulator.  The  sec- 
ondary is  divided  into  a  number  of  coils  which  can  be 
inserted  or  removed  from  the  circuit,  according  to  the 
amount  of  variation  of  voltage  desired.  A  reversing 
switch  is  provided,  so  that  the  E.M.F.  generated  in  the 


162    POLYPHASE  APPARATUS  AND  SYSTEMS. 


regulator  can  be  either  added  to  or   subtracted  from  the 
pressure  of  the  feeder. 

Some  of  the  important  uses  to  which  pressure  regulators 
can  be  put  are  the  following :  For  regulating  the  voltage 

of  alternating-current 
feeders  ;  for  equalizing 
voltage  on  unbalanced 
polyphase  circuits  ;  as 
dimmers  for  theatres  ; 
as  regulators  for  se- 
ries-alternating cir- 
cuits, either  for  arc  or 
incandescent  lights. 

The  rating  in  watts 
is  the  product  of  the 
secondary  current 
in  amperes,  by  the 
boosting  capacity  in 
volts. 

Rectifiers.  —  A  rec- 
tifier is  a  device  for 
changing  an  alternat- 
ing current  into  a  di- 
rect current,  and  is 
intended  mainly  for 
the  operation  of  series 
arc  lamps.  The  ap- 
paratus usually  con- 
sists of  a  constant- 
current  transformer,  giving  constant  alternating  current  at 
all  loads,  and  a  rectifying  device  which  runs  in  synchro- 
nism with  the  alternating,  and  converts  the  constant 


Fig-.  1O8. 


STATION   EQUIPMENT  AND  APPARATUS.      163 

alternating  current  into  a  direct  current,  but  more  or  less 
pulsating. 

At  light  loads  the  rectifier  has  an  idle  current  of  nearly 
100  per  cent  of  full-load  current,  and  at  full  load  a  low 
power-factor.  For  polyphase  circuits,  therefore,  to  avoid 
unbalancing  of  the  phases,  the  rectifier  should  preferably 
be  of  polyphase  design. 

Frequency  Changer.  —  In  alternating-current  plants,  em- 
ploying a  low  frequency,  there  is  sometimes  a  need  for  a 
limited  amount  of  current  of  a  higher  frequency.  For 
instance,  in  25  and  40  cycle  installations,  incandescent  and 
arc  lighting  may  be  required.  To  meet  such  cases  a  fre- 
quency of  60  cycles,  or  any  other  number  of  cycles  suitable 
for  lighting,  may  be  obtained  most  economically  and  cheaply 
by  means  of  a  frequency  changer.  This  is  essentially  an 
induction  motor,  the  armature  of  which  is  rotated  by  a 
synchronous  motor  in  a  direction  usually  opposite  to  its 
natural  rotation.  The  lower  frequency  current  is  fed  to 
the  primary  or  field,  and  the  current  at  the  higher  fre- 
quency is  taken  out  of  the  secondary  or  armature  by  means 
of  collector  rings.  The  frequency  and  voltage  of  the  out- 
put will  depend  on  the  speed  of  the  secondary,  and  will  be 
the  algebraic  sum  of  the  current  pulsations  in  both  mem- 
bers. If  the  secondary  is  run  at  rated  speed,  but  in  oppo- 
sition to  its  natural  rotation,  the  frequency  will  be  twice 
that  of  the  normal  current,  or  if  run  at  one-half  speed  in 
its  natural  direction,  the  frequency  will  be  one-half  the 
normal.  To  change  a  frequency  of  40  cycles  to  60  cycles, 
the  secondary  would  be  run  at  one-half  speed  in  an  oppo- 
site direction,  while  to  obtain  60  cycles  from  a  25  cycle 
current,  the  secondary  would  run  nearly  two  and  one-half 
times  the  rated  speed  in  an  opposite  direction. 


1 64   POLYPHASE  APPARATUS  AND  SYSTEMS. 

The  capacity  of  the  driving-motor  end  of  the  frequency 
changer  bears  the  same  proportion  to  the  total  output  that 
the  increase  in  frequency  bears  to  the  final  frequency. 
The  secondary  of  the  frequency  changer  proper  must  equal 
the  output.  The  capacity  of  the  primary  has  the  same 
proportion  to  the  total  output  that  the  initial  frequency  has 
to  the  final  frequency.  As  an  illustration, —  a  100  K.W. 


\\ 


Fig.  1O9. 

frequency  changer,  primary  40  cycles,  secondary  60  cycles, 
would  be  composed  as  follows  :  A  40  cycle  synchronous 
motor,  capacity  33  K.W.,  speed  600  R.P.M.,  direct-con- 
nected to  the  secondary,  capacity  100  K.W.  Primary  capa- 
city would  be  66  K.W.  The  primary  would  be  four  polar. 
The  natural  speed  of  the  secondary  would  be  1,200  R.P.M. 
By  driving  it  at  a  speed  of  600  R.P.M.  in  the  opposite 
direction  to  its  natural  rotation,  the  number  of  reversals 


STATION    EQUIPMENT   AND   APPARATUS.       165 

will  be  that  due  to  an  equivalent  speed  of  1,800  R.P.M.,  or 
60  cycles.  For  the  sake  of  illustration,  the  capacities  as 
given  above  are  on  the  assumption  of  a  100  per  cent  effi- 
ciency, which,  of  course,  is  an  impossibility.  Fig.  109 
depicts  the  general  form  of  this  apparatus. 

Motor  Generators. — A  motor  generator  for  alternating- 
current  work  consists  of  an  induction  or  synchronous 
motor,  mounted  on  the  same  base  and  direct  connected  to 
a  generator.  It  may  perform  the  functions  of  a  frequency 
changer,  in  which  case  the  generator,  of  course,  is  of  the 
alternating-current  type,  or  it  may  be  used  in  place  of  a 
rotary  converter,  the  generator  then  delivering  a  direct 
current.  Although  more  expensive  and  less  efficient  than 
a  rotary  converter,  the  motor  generator  has  the  advantage 
of  not  always  requiring  step-down  transformers.  It  is  self- 
regulating,  and  is  not  materially  affected  by  potential  fluc- 
tuations of  the  transmission  lines. 


166      POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER   IX. 
TWO-PHASE  SYSTEM. 

Polyphase  Systems  and  Combinations.  —  Any  arrange- 
ment of  conductors,  carrying  two  or  more  single-phase 
alternating  currents,  definitely  related  to  one  another  in 
point  of  time,  constitutes  a  polyphase  system.  The  sys- 
tems commonly  employed  for  the  generation  and  distribu- 
tion of  power  by  polyphase  currents  are  the  two-phase, 
three-phase,  and  a  third  system,  which  is  a  combination  of 
a  single-phase  and  polyphase  conductor  arrangement,  called 
the  monocyclic  system. 

Polyphase  currents  are  usually  produced  by  generators, 
the  armatures  of  which  are  so  wound  that  the  electro- 
motive forces  at  the  terminals  correspond  to  the  number 
of  phases,  and  arrive  at  a  maximum  in  a  fixed  and  definite 
relation  to  one  another. 

In  the  two-phase  system  the  two  electro-motive  forces 
and  currents  are  90°,  or  one-fourth  of  a  cycle,  apart.  The 
relations  of  the  curves  to  each  other,  and  their  instantaneous 
values,  can  be  seen  from  the  development  of  the  diagram  of 
single  harmonic  motion  (Fig.  no).  The  maximum  of  one 
wave  occurs  when  the  value  of  the  other  is  zero.  If  the 
pressure  in  any  one  of  the  coils  Oa  or  Ob  is  I,  the  pressure 
between  the  ends  ab  is  \f~2=  1.414. 

The  windings  of  a  polyphase  machine  may  be  combined 
in  a  number  of  ways,  each  affecting  the  relation  of  the 


TWO-PHASE   SYSTEM. 


electro-motive  forces  of  the  outside  conductors,  as  shown 
in  Figs,  in  to  114.  These  diagrammatically  represent 
the  coils  of  a  two-phase  machine,  in  which  the  electro- 


lotive  forces  may  be  considered  as  being  either  generated 
absorbed.  In  Fig.  1 1 1  all  the  coils  are  in  series,  form- 
a  continuous  winding,  tapped  at  four  points.  This 
rangement  is  known  as  an  interlinked  winding.  Leads 
and  2  constitute  the  circuit  of  one  phase,  and  3  and  4 
lat  of  the  second  phase.  The  E.M.F.  between  the  wires 
of  different  phases  is  1 .4  times  that  between  leads  of  the 
same  phase.  In  Fig. 
1 1 2  the  windings  of 
each  phase  are  sepa 
rate.  This  arrange- 
ment can  be  made  in- 
terlinked by  joining  the 
two  circuits  where  they 
cross,  thus  forming  a 
common  centre,  as 
shown  in  Fig.  113. 
The  relation  of  E.M.F. 


Pig-.  111. 


is  the  same  as  in  Fig.  112.  The  grouping  of  coils,  shown 
in  Fig.  1 12,  may  also  be  made  interlinked  by  joining  leads 
3  and  4  (Fig.  114),  which  become  a  common  return  for  I 


l68       POLYPHASE  APPARATUS  AND   SYSTEMS. 

and  2.  The  E.M.F.  between  the  two  outgoing  wires  is  1.4 
times  that  between  each  outgoing  wire  and  the  common 
return. 

The  windings  of  interlinked  systems  are  classed  accord- 
ing to  their  connections  as  "Ring,"  or  "  Star."  Figs,  in 
and  1 1 3  respectively,  show  the  ring  and  star  connections  of 
the  two-phase  system. 

In  the  three-phase  system,  the  ring  and  star  connections 


Fig.   114. 

are  usually  designated  as  Y  and  A  (Delta),  from  their 
resemblance  to  these  symbols. 

The  winding  connections  of  most  commercial  two-phase 
machines  are  interlinked.  Fig.  1 1 5  shows  the  connections 
of  a  Westinghouse  two-phase  2,000  volt  generator.  Con- 
nections are  made  to  the  winding  at  four  points. 

The  current  in  the  circuit  1-3  is  90°  apart  from,  or  in 


TWO-PHASE   SYSTEM. 


169 


ITT 

t 

Q 

| 

t               "  \ 

( 

fl|| 

O               Q 

1   ! 

1                   '\ 

170   POLYPHASE  APPARATUS  AND  SYSTEMS. 

quadrature  with,  the  current  in  the  circuit  2-4.  The  E.M.F. 
existing  between  any  two  adjacent  terminals  is  1,400  volts. 
If  the  E.M.F.  is  raised  or  lowered,  the  same  proportions 
hold;  and  for  a  1,000  volt  machine,  the  electro-motive 
forces  are  respectively  1,000  and  700  volts. 

No  matter  what  the  arrangement  of  the  winding  may 
be  in  a  polyphase  machine,  whether  the  coils  are  inter- 
linked, or  separately  grouped,  ring  or  star  connected,  the 
principles  of  action  are  the  same,  and  the  characteristic 
polyphase  results  are  equally  present. 

Polyphase  systems  have  two  desirable  features  :  First, 
the  supply  of  power  is  continuous  and  uniform,  thus 
increasing  the  capacity  of  apparatus,  and  in  some  systems, 


100O 

i 


<>           I 
/         1QO 

>             1 

1 

3 

<*          100 

<•    I 

2 
4 

Fig.    116. 

that  of  transmission  conductors  ;  and,  second,  the  use  of 
revolving  types  of  induction  apparatus  is  permitted,  which 
do  not  require  any  form  of  moving  contacts. 

Transformer  Connections.  —  A  number  of  combinations 
of  two-phase  circuits  can  be  made  by  suitably  arranging 
transformers  with  due  regard  to  the  generator  windings. 
Fig.  116  shows  the  connections  commonly  used  for  light- 
ing and  transmission  of  power.  The  arrangement  consists 
of  two  single-phase  transformers,  the  phases  being  sepa- 
rated in  both  primary  and  secondary.  Two  of  the  sec- 
ondary leads  are  sometimes  joined  (Fig.  1 1 7),  making  a 


TWO-PHASE  SYSTEM. 


171 


common  return  for  the  other  wires.  The  two  circuits  being 
90°  apart,  the  voltage  between  I  and  4  is  V2  times  that 
between  the  outside  wires  and  the  common  return.  This 
arrangement  is  best  adapted  for  supplying  current  of  mini- 
mum potential  to  apparatus  in  the  vicinity  of  the  trans- 
formers. It  is  more  frequently  used  in  connection  with 
motors  operating  from  the  secondaries  of  the  transformers. 


S     a     i< 

1  ! 

10 

I                 af 

2    1 

1000       B 


b    100 


00 

I 4    I 


Fig-.  117. 


1 

A 
a     I 

k* 

| 
10JOO 

V 

\ 

2 

1<00 

B     i 

4      1 

* 
io]oo 

\ 

I 

100 


Pig.   118. 

Fig.  1 1 8  shows  another  arrangement  of  transformers  where 
the  common  return  is  used  on  both  primary  and  secondary. 
As  will  be  explained  farther  on,  this  connection  is  permis- 
sible only  when  the  power  of  the  two  circuits  is  consumed 
by  one  unit,  or  when  both  sides  of  the  system  are  bal- 
anced. 

Two-Phase  to  Three-Phase.  —  It  is  possible,  by  a  combi- 
nation of  two  transformers,  to  change  one  polyphase  sys- 
tem into  any  other  polyphase  system.  The  transformation 


POLYPHASE  APPARATUS  AND  SYSTEMS. 


from  two-phase  to  three-phase,  or  vice  versa,  is  effected 
by  proportioning  the  windings,  as  shown  in  Fig.  1 19.  One 
transformer  is  wound  with  a  ratio  of  transformation  of 
1,000  to  100;  the  other  with  a  ratio  of  1,000  to  86.7.  The 
secondary  of  this  transformer  is  connected  to  the  middle 
of  the  secondary  winding  of  the  first.  In  Fig.  120,  AB 

represents  the 


A      qpronrlqrv  volts 

10 

\  C3  t 

from  A  to  B  in 
D     one     transfor- 

1C 

0 

K 

mer.     At  right 
angles  to    AB 
r    the  line  CO  re- 

10 

00                                >      S  86.7 

Fig<  119>                                   presents,  in  di- 
rection    and 

quantity,  the  pressure  O  to  C  of  the  second  transformer. 
From  the  properties  of  the  triangle  it  follows  that,  at  the 
terminals  A,  B,  C,  three  equal  pressures  will  exist,  each 
differing  from  the  others  by  60  ,  and  giving  rise  to  a  three- 
phase  current. 

For  this  transformation  on  a  small  scale,  it  is  customary 
to   use   standard  transformers,   the 
main   transformer  having  a  ratio  of 
10  to  i,  and  the  teaser  a  ratio  of 
9  to  i. 

The  current  in  the  winding  OC, 
being  a  resultant  of  the  other  two 
phases,  is  greater  than  if  the  change 
to  three-phase  were  not  made;  and, 
consequently,  for  the  same  heating, 
necessitates  more  transformer  capacity.  Only  one  trans- 
former, the  teaser,  need  be  of  greater  output.  The  increase 


TWO-PHASE   SYSTEM. 


173 


is  in  the  secondary,  being  1 5  per  c£nt,  or  about  4  per  cent 
of  the  total  transformer  capacity.  If  the  transformers  are 
interchangeable,  the  excess  capacity  required  in  the  two 
transformers  is  over  12  per  cent.  The  secondary  of  each 
interchangeable  transformer  has  two  taps,  giving  50  per 
cent  and  86.7  per  cent  of  the  full  voltage,  so  that  either 
transformer  can  serve  as  the  teaser,  or  supplementary  one, 
by  using  the  proper  terminals. 

In  the  long-distance  transmission  of  power  the  genera- 
tors are  sometimes  wound  two-phase,  arid  the  secondary 


rwvwi    s  < 

Fig.  121. 

distribution  at  the  receiving  end  is  likewise  by  the  two- 
phase  system,  while  on  account  of  the  saving  in  copper  the 
transmission  is  by  the  three-phase  system.  Such  is  the  ar- 
rangement of  the  apparatus  at  the  generating  end  of  the 
Niagara-Buffalo  plant.  The  distribution  in  Buffalo,  how- 
ever, is  mainly  by  the  three-phase  system.  Fig.  121  shows 
the  transformer  connections  for  changing  two-phase  to 
three-phase  and  back  again. 

Two-Phase  Four- Wire  System This   system   consists 

of  two  separate  circuits,  derived  from  two  independent 
armature  windings  in  quadrature  with  each  other,  or  from 
a  continuous  armature  winding  tapped  at  four  equidistant 


1/4   POLYPHASE  APPARATUS  AND  SYSTEMS. 

points.  The  practical  application  of  this  system  is  illus- 
trated in  Fig.  122.  Each  of  the  two  generators  A  and  B 
delivers  two-phase  currents  of  low  potential  to  the  step-up 
transformers  RT,  RT',  RT",  RT'",  through  the  switch- 
board  D.  The  transmission  lines  Z,  L',  L",  L!",  receive  and 
transmit  current,  at  a  high  pressure,  to  a  substation  con- 
veniently located  with  reference  to  the  districts  where  lights 
and  motors  are  to  be  supplied.  The  high-potential  current 
is  here  reduced  by  the  transformers  L  T,  L  T',  L  T",  L  T"1,  to 
a  commercial  pressure  suitable  for  local  distribution,  through 
the  switchboard  F.  Beginning  at  the  right  of  the  figure, 
the  first  four-wire  system  is  used  to  supply  alternating  cur- 
rent to  the  rotary  converter,  which,  in  turn,  delivers  direct 
current  at  500  volts  to  a  trolley  line  operating  the  street- 
car systems  K.  The  second  circuit  supplies  the  motors  M, 
M',  M",  M'",  either  of  the  synchronous  or  induction  type. 
The  next  four-wire  system  is  divided  into  two  distinct 
circuits,  supplying  current  to  incandescent  lamps  through 
the  transformers  b,  b',  B",  B'".  The  next  circuit  supplies 
current  for  arc  lighting  through  a  rotary  converter.  An- 
other rotary  converter  is  operated  from  the  last  circuit, 
and  delivers  low-voltage  current  for  electrolytic  purposes. 
The  rotary  converters  in  practice  are  supplied  with  trans- 
formers, not  shown  in  the  diagram,  which  deliver,  at  the 
rotary  terminals,  an  alternating  current  of  the  proper  vol- 
tage. 

The  two  single  circuits  must  be  balanced  as  nearly  as 
possible,  and  for  this  purpose  the  four  wires  must  be  car- 
ried through  the  same  district  to  be  supplied  with  power 
or  light.  In  order  to  obtain  economy  in  copper  in  a  sec- 
ondary system  of  distribution,  it  is  desirable  to  use  three- 
wire  mains.  In  the  two-phase  four-wire  system,  where 


TWO-PHASE    SYSTEM, 


175 


1/6        POLYPHASE    APPARATUS    AND    SYSTEMS. 

motors  are  to  be  supplied,  the  two  independent  three- 
wire  circuits  must  be  brought  together,  making  six  wires 
in  all. 

The  measurement  of  power  by  this  system  is  obtained 
by  the  use  of  a  wattmeter  inserted  in  each  circuit,  as  in  a 
single-phase  system.  The  sum  of  the  two  readings  gives 


WW 
wv< 


VMTWW 


/w 


-o— 


J 


Fig.  123. 

the  total  power  supplied.      In  a  balanced  system,  twice  the 
reading  of  one  wattmeter  will  give  the  power. 

Two-Phase  Three-Wire  System.  —  By  joining  any  two 
conductors  in  the  four-wire  system,  a  common  return  is 
made  for  the  two  circuits.  This  arrangement  of  circuits 
is  called  the  two-phase  three-wire  system.  As  previously 
shown,  the  pressure  between  the  common  conductor  and 


TWO-PHASE   SYSTEM.  177 

the  others  is  42  per  cent  higher  than  that  //nich  existed 
before.  With  a  given  load  and  insulation  strain,  the  com- 
mon conductor  must  be  made  larger  in  pn  portion,  in  order 
to  keep  the  loss  the  same. 

The  general  application  of  this  system  is  shown  in  Fig. 
123.  Two  terminals  of  the  generator  coils  are  united;  and 
the  three  leads,  forming  an  interconnected  two-phase  sys- 
tem, are  run  to  wherever  motors  and  lights  are  to  be  sup- 
plied. When  motors  are  used,  connection  is  made  directly 
with  the  main  leads,  or,  if  the  motors  are  wound  for  low 
voltage,  connection  is  made  through  two  transformers. 
The  motors,  which  are  of  the  ordinary  two-phase  type,  may 
have  their  terminals  connected  either  on  the  three-wire  or 
four-wire  system. 

Where  lights  are  supplied,  the  transformers  may  be  con- 
nected singly  to  only  one  circuit,  or  in  pairs  on  two  cir- 
cuits, with  a  common  return.  In  practice,  it  is  essential 
that  both  phases  be  equally  loaded. 

In  this  arrangement  of  conductors  there  is  an  unbalan- 
cing of  both  sides  of  the  system  on  an  inductive  load,  which 
exists,  even  though  the  energy  load  is  equally  divided. 
This  unbalancing  is  due  to  the  fact  that  the  E.M.F.  of  self- 
induction  in  one  side  of  the  system  is  in  phase  with  the 
effective  E.M.F.  in  the  other  side,  thus  distorting  the  uni- 
form current-distribution  in  both  circuits. 

The  distribution  of  currents  and  electromotive  forces  in 
the  three  conductors  in  the  single-phase  three-wire,  the 
three-phase  and  the  two-phase  three-wire  system,  is  shown 
in  the  following  table.  The  figures  are  the  results  of  ex- 
periments to  determine  the  self-induction  of  underground 
tubes, 


178      POLYPHASE  APPARATUS  AND   SYSTEMS. 


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TWO-PHASE   SYSTEM.  179 

The  single-phase  and  the  three-phase  systems  give  equal 
drops,  but  the  induction  unbalancing  of  the  two-phase 
three-wire  system  is  beyond  the  range  of  practical  opera- 
tion. These  results  were  obtained  with  low-tension  sys- 
tems and  moderate  drops.  The  unbalancing  effect  is  much 
greater  with  higher  voltage  and  drops.  The  four-wire  two- 
phase  system  would,  of  course,  show  no  such  unbalancing. 

The  two-phase  system  is  admirably  adapted  for  lighting 
distribution  when  the  two  circuits  are  not  connected.  In 
places  having  already  single-phase  wiring,  the  change  to 
polyphase  would  often  require  considerable  and  expensive 
alterations  with  any  system  but  the  two-phase. 


180       POLYPHASE  APPARATUS   AND   SYSTEMS. 


CHAPTER    X. 
THREE-PHASE  SYSTEM. 

Curves  of  E.M.F. — The  E.M.R  impulses  in  a  three- 
phase  system  follow  one  another  at  intervals  of  60°.  The 
instantaneous  values  and  the  relation  of  phases,  developed 
from  the  diagram  of  simple  harmonic  motions,  are  shown 
in  Fig.  124.  The  curves  Oa,  Ob,  Oc,  represent  the  electro- 
motive forces  produced  by  three  sets  of  generator  coils.  If 
the  distance  from  O  to  ay  b,  and  c,  be  taken  equal  to  i,  it 


Fig.  124. 

follows  from  the  diagram  that  the  lines  joining  a,  b,  and  c 
are  equal  to  V  3  =  r-732-  That  is,  the  pressure  between 
the  ends  of  any  two  of  the  generator  coils  in  a  three-phase 
system  is  1.732  times  that  between  the  common  juncture 
O  and  the  terminals  of  the  coils. 

It  will  be  seen  from  the  diagram  that  each  one  of  the 
coils  successively  serves  as  a  return  for  the  other  two,  and 
that  the  algebraic  sum  of  the  currents  in  the  system  is 


THREE-PHASE   SYSTEM. 


181 


zero.  The  three-phase  system  may  be  resolved  into  three 
single  circuits,  with  a  common  or  grounded  return.  The 
sum  of  the  currents  being  zero,  no  current  will  flow  in  the 
return  conductor,  and  it  may  be  dispensed  with.  The 
system  then  becomes  the  ordinary  F-connected  arrange- 
ment. 

Transformer  Combinations.—  The  ring  and  the  star  con- 
nections of  three-phase  windings,  —  whether  of  armature  in 


PRIMARY 


SECONDARY 


Fig.  125. 


which  the  electromotive  forces  are  induced,  or  transformer, 
or  motor  in  which  the  electromotive  forces  are  absorbed, 
—  are  designated  by  the  symbols  A  and  Y  respectively. 
Figs.  125  to  129  illustrate  the  various  three-phase  combi- 
nations of  single-phase  transformers  in  practical  operation. 
Fig.  125  shows  A  connection  of  both  primary  and  secon- 
dary terminals  of  transformers,  having  a  ratio  of  10  to  i. 
Fig.  126  shows  three  transformers,  Y  connected  in  both 
windings.  The  ratio  of  pressures  between  any  two  corre- 
sponding terminals  in  primary  and  secondary  are  the  same 
as  in  the  A  arrangement,  The  individual  transformers 


182       POLYPHASE  APPARATUS   AND   SYSTEMS. 

thus  connected  have  fewer  turns  for  the  same  voltage  than 
when  A  connected,  and  thus  this  arrangement  is  suitable  for 
very  high  line-pressures.  Fig.  127  shows  a  combination  of 
A  and  Y  connection,  the  primaries  of  the  transformers  being 


PB1MARY 


SECONDARY 


Fig.  126. 


PRIMARY 


SECONDARY 


Fig.  127. 

connected  A,  while  the  secondaries  are  connected  Y.  A 
fourth  wire  may  be  led  from  the  common  centre  of  the 
three  secondaries.  The  pressure  between  this  neutral  and 

one  of  the  outside  wires  is  —•=•  of  the  pressure  between 


THREE-PHASE   SYSTEM. 


183 


the  outside  wires.  This  arrangement  is  known  as  the 
"three-phase  four-wire  system,"  and  is  especially  conven- 
ient and  economical  in  secondary  distributing  systems.  In 


PRIMARY 


SECONDARY 


Fig.  128. 


PRIMARY 


SECONDARY 


Fig.  129. 


Fig.  128,  the  primaries  are  connected  Y,  the  secondaries 
A.  The  A  connection  is  sometimes  made  up  of  two  trans- 
formers (Fig.  129),  instead  of  three.  The  pressures  be- 


184   POLYPHASE  APPARATUS  AND  SYSTEMS. 

tween  all  three  terminals  are  equal,  that  from  the  open 
side  of  the  triangle  being  the  resultant  of  the  E.M.F.  in 
the  existing  windings.  This  arrangement  is  frequently 
used  with  motors,  its  chief  advantages  being  its  simplicity, 
and  permitting  the  use  of  available  transformers,  when  the 
motor  cannot  be  fitted  with  three  transformers  of  exactly 
the  capacity  wanted.  Its  disadvantage  is  that  the  motor 
will  stop  in  case  of  accident  to  one  transformer.  The  com- 
bination of  three  transformers,  arranged  in  A,  is  most  con- 
venient and  desirable,  for  the  reason  that  an  accident  to 


Generator 


one  does  not  interrupt  the  service  ;  the  only  requirement 
being,  that  the  load  be  reduced  one-third,  to  prevent  heat- 
ing of  the  transformers.  Another  disadvantage  of  the  re- 
sultant A  arrangement  is  the  increased  transformer  capacity 
required,  as,  for  the  same  total  energy,  the  flow  of  current 
is  increased  through  the  two  existing  secondaries.  This 
disadvantage  is  not  so  noticeable  in  small  transformers,  but 
must  be  allowed  for  when  working  with  large  transformers. 

Motor  Connections.  —  Motors  are  connected  to  the  sec- 
ondaries of  three  transformers  in  a  three-phase  system,  as 
shown  in  Fig.  130. 

The  primaries,  p- 1,  /-2,/-3,  of  three  transformers  are 


THREE-PHASE   SYSTEM.  185 

connected  between  the  three  lines  A,  B,  C,  leading  from 
the  generator,  and  three  secondaries,  5-1,  S-2,  5-3,  are 
connected  in  delta  to  the  three  lines  a,  b,  c,  leading  to  the 
motor.  A  recording  wattmeter  of  the  three-phase  type, 
for  measuring  the  power  consumed  by  the  motor,  is  shown 
connected  in  the  system  with  the  field  spools  at  /,  the 
armature  circuit  a'  and  its  resistances  r,  between  the  three 
secondary  lines. 

Induction  motors  may  be  supplied  from  a  three-phase 
generator  by  means  of  two  reducing  transformers  in  the 
manner  shown  in  Fig.  131.  This  arrangement  is  identical 


Q 
Generator  ^^  ^J  Motor 


C  c 

Fig.  131. 

with  that  in  Fig.  130,  except  that  one  of  the  transformers, 
P-3,  5-3,  is  left  out,  and  the  two  other  transformers  are 
made  correspondingly  larger.  The  recording  wattmeter  is 
connected  in  the  secondary  circuit  in  the  same  way  as  in 
the  use  of  three  transformers. 

The  connections  of  three  transformers  for  a  low-tension 
distribution  system,  by  the  three-phase  four-wire  system, 
are  shown  in  Fig.  132.  The  three  transformers  have  their 
primaries,  P-i,  P-2,  P-$,  joined  in  delta  connection,  and 
their  secondaries,  5-1,  5-2,  5-3,  in  Y  connection.  Lines 
a,  b,  c,  are  the  three  main  three-phase  lines,  and  d  is  the 
common  neutral.  The  difference  of  potential  between  a 
and  bj  b  and  c,  and  a  and  c  is  200  volts,  while  that  between 


186   POLYPHASE  APPARATUS  AND  SYSTEMS. 

them  and  d  is  115  volts.  200  volt  motors  are  joined  to 
a,  by  and  c,  while  1 1 5  volt  lamps  are  connected  between  a 
and  d,  b  and  d,  or  c  and  d.  Line  d,  like  the  neutral  wire 
in  the  Edison  three-wire  system  only  carries  current  when 
the  lamp  load  is  unbalanced. 


Generator 


>            a 

5:  s1 
^  

20\)  V 

^  S2 

>• 

! 

6 

j 

|     20p  V 

Tl 

g  S3 

'        C          T           '     1  15  V 
1  1!5  V 

nr  i  1 

Fig.  132. 

Measurement  of  Power.  —  In  a  F  connected  generator  the 

E.M.F.,  induced  in  each  phase,  is  -  ^ ,  and  the  energy  in 

V3 

that  phase  is  7  x  —=.  >  E  being  the  E.M.F.  at  the  generator 

V3 
terminals.     In  a  A  connected  generator  the  current  in  each 

phase  is  -= ,  7  being  the  line  current,  and  the  energy  is 

V3 

E  x  — —  .     The  total  energy  for  the  three  phases,  in  the 

v3 

cases  both  of  a  Fand  a  A  connected  generator,  is  =  V^3  x  E 
X  7.  This  formula  is  correct  when  the  generator  output  is 
of  a  non-inductive  character.  If  a  phase  displacement  ex- 
ists, the  expression  becomes  V  3  x  E  x  7  x  Cos  <£.  These 
formulas  apply  equally  well  for  determining  the  power  in  a 
three-phase  circuit,  irrespective  of  the  method  of  connec- 
tions of  the  supplying  source  or  of  the  consuming  devices. 


THREE-PHASE  SYSTEM.  187 

As  an  illustration,  —  the  power  in  a  non-inductive  three- 
phase  circuit,  in  each  branch  of  which  100  amperes  is  flow- 
ing, the  voltage  between  lines  being  2,500,  is  found  as 
follows:  the  energy  in  each  phase  is  =  100  amperes  x  2,500 

volts  x  -==145  K.W.,  and,  for  the  three  circuits,  is  there- 

V3 

fore  435  K.W.     If  the  circuit  had  a  power  factor  of  80 
per  cent,  the  energy  would  then  be  435  x  .80  =  348  K.W. 

The  power  supplied  by  three-phase  circuits  can  be  meas- 
ured by  the  use  of  three,  two,  or  one  wattmeter.  Three 
wattmeters  will  give  the  power  of  a  circuit  irrespective  of 
the  condition  of  balancing  or  lag.  The  sum  of  the  read- 
ings of  the  three  instruments  is  the  total  power.  Each 
wattmeter  must  be  connected  to  the  common  centre  or 
neutral  of  the  system.  If  the  apparatus  is  connected 
delta,  it  is  necessary  to  make  an  artificial  neutral  with 
resistances.  Two  wattmeters  can  be  connected  so  that,  as 
long  as  the  power  factor  is  greater  than  50  per  cent,  the 
sum  of  the  two  readings  equals  the  total  power.  The  dif- 
ference of  the  two  readings  will  give  the  power  when  the 
power  factor  is  less  than  50.  As  it  is  not  possible  to  tell 
when  the  power  factor  falls  below  this  point,  without 
reversing  the  connections,  this  method  is  inconvenient  and 
undesirable. 

The  usual  method  of  measuring  power  in  three-phase 
circuits  is  by  one  wattmeter.  Three-phase  circuits  when 
interlinked  are  easily  kept  in  balance  in  respect  to  load  and 
power  factor.  Three  times  the  readings  of  the  single 
wattmeter  will  give  the  total  power  in  the  circuits,  if  they 
are  balanced.  Figs.  133  and  134  show  the  connections  of 
three-phase  recording  wattmeters  for  low  and  for  high 
voltage  circuits.  The  wattmeter  is  provided  with  resist- 


188      POLYPHASE  APPARATUS  AND   SYSTEMS. 


To  Generator 


110-220-550  Volts 
Fig.  133. 


To  Line 


To  Generator 


j — a 


/ 150-2300  Volts 
Fig.  134. 


To  Line 


THREE-PHASE   SYSTEM. 


I89 


ances,  rr  and  r1,  for  creating  an  artificial  neutral.  The 
armature  windings  are  in  series  with  /,  so  that  r'+a  = ;-. 
The  wattmeter,  diagrammatically  illustrated  in  Fig.  133, 


Fig.  135. 


is  adapted  for  circuits  of  550  volts  and  less.  Fig.  134 
shows  the  connection  of  a  voltmeter  for  circuits  of  from 
1,000  to  3,000  volts.  Station  transformers  t  and  t  are 
required  to  reduce  the  pressure  for  the  armature  windings. 


190       POLYPHASE   APPARATUS   AND    SYSTEMS. 

The  method  of  installing,  and  the  connections  for  this  watt- 
meter transformer,  are  illustrated  by  Fig.  135. 

The  connections  for  an  indicating  wattmeter  are  the 
same  as  those  for  a  recording  wattmeter.  The  main  cur- 
rent is  taken  by  the  stationary  or  low-resistance  coil,  while 
the  pressure  coil  is  of  high  resistance,  and  connected  to 
the  artificial  neutral. 

Three-Phase  Circuits.  —  The  general  arrangement  of 
circuits  for  a  local  distribution  of  light  and  power  is 
shown  in  Fig.  136.  The  generators  are  wound  for  2,000 
volts,  feeding  direct  into  the  mains.  Step-down  trans- 
formers reduce  the  power  to  100  volts  for  light  and  200 
volts  for  motors.  In  one  arrangement  alternating  enclosed 
arc  lights  are  shown,  operated  from  a  transformer.  A  200 
volt  motor  is  supplied  by  three  transformers,  constituting 
a  system  of  secondary  mains.  In  the  second  arrangement 
a  motor  running  from  two  transformers,  and  a  general  dis- 
tributing system,  are  shown.  The  general  practice  is  to 
wind  the  generators  for  1,040  or  2,080  volts,  no  load,  and 
use  transformers  reducing  to  1 1 5  volts  for  lights  and  small 
motors.  Where  secondary  mains  are  employed  the  motor 
pressure  is  200,  220,  440,  or  550  volts. 

Where  lights  and  motors  are  located  a  considerable  dis- 
tance from  the  generators,  the  cost  of  copper  is  reduced  by 
employing  transformers  to  raise  the  current  pressure.  An 
arrangement  of  three-phase  circuits  for  transmitting  power 
over  long  distances  is  shown  in  Fig.  137.  The  generator, 
direct-connected  to  the  source  of  power,  a  water-wheel,  is 
shown  at  A.  B  is  a  bank  of  step-up  transformers,  rais- 
ing the  voltage  to,  say,  20,000.  As  this  voltage  is  higher 
than  can  be  used  with  any  apparatus  for  direct  utilization 
of  the  current,  step-down  transformers,  CIt  C2,  and  C3,  are 
required. 


THREE-PHASE    SYSTEM.  IQI 

The  main  substation  contains  the  transformers,  Cl  and 
C9.  This  is  a  true  central  or  distributing  station.  From 
this  point  the  distributing  feeders  are  taken  out  at,  say, 
2,080  volts,  for  the  commercial  primary  circuits  and  through 
the  bank  C,  at  115  volts,  to  feed  a  low-tension  network. 
Through  the  transformer  C1}  a  current  of  2080  volts  is 
fed  direct  into  a  synchronous  motor,  and  into  transformers 


Fig.  136. 

reducing  to  115  volts  for  supplying  motors  and  lights. 
The  substation  transformers  C2,  furnish  current  for  a 
general  lighting  and  motor  service  at  /,_/,  and  H.  The  vol- 
tage for  this  distributing  system  is  controlled  by  the  reg- 
ulators G. 

At  C3  another  bank  of  step-down  transformers  is 
located.  An  alternating  current  of  suitable  voltage  is  de- 
livered to  the  rotary  converter  D,  which  supplies  contin- 


IQ2  .     POLYPHASE   APPARATUS   AND   SYSTEMS. 

uous  current  to  the  electrolytic  vats  or  storage  battery  E. 
A  rotary  might  also  furnish  direct  current  for  electric  rail- 
way service. 


A  modified  three-phase  system,  in  which  the  lighting 
service  is  supplied  from  one  branch  of  the  system,  has 
been  used  by  Mr.  Steinmetz,  and  designated  as  the  poly- 


THREE-PHASE   SYSTEM. 


193 


cyclic  system  (Fig.   138).     It  is  practically  a  single-phase 
system  for  lighting  service  and  three-phase  for  motor  work. 


Fig.  138. 


This  system  may  be  used  when  the  lighting  load  is  not 
over  25  or  30  per  cent  of  the  total  load.  The  lighting 
circuit  is  perfectly  balanced  for  all  loads. 


194       POLYPHASE  APPARATUS   AND   SYSTEMS. 


CHAPTER  XL 
MONOCYCLIC  SYSTEM. 

General.  —  The  two-phase  and  three-phase  systems  have 
made  possible  the  transmission  of  power  over  long  dis- 
tances. These  systems  also  possess  notable  advantages  in 
the  local  distribution  of  power  involving  large  units,  and 
in  many  special  applications.  For  general  central  station 
work,  such  as  the  distribution  of  alternating  currents  for 
lighting,  with  incidental  power,  the  monocyclic  system  de- 
signed by  Mr.  C.  P.  Steinmetz  is  especially  suited. 

The  monocyclic  system  is  essentially  a  single-phase  sys- 
tem, consisting  of  two  wires  in  combination  with  a  third 
auxiliary,  or  teaser  wire  ;  the  main  lines  being  used  for  sup- 
plying lights,  while  the  third  wire,  which  carries  an  inter- 
mediate, or  displaced,  current,  is  used,  together  with  the 
main  lines,  for  supplying  power  to  polyphase  motors.  The 
teaser  wire  need  only  be  run  to  the  motors.  Indeed, 
the  teaser  wire  need  not  start  from  the  generator,  but  may 
start  from  any  motor,  or  multiple-circuit  apparatus,  of  the 
system.  The  motors  operate  practically  the  same  as  poly- 
phase motors.  As  the  lights  are  connected  to  the  single- 
phase  circuit,  there  is  no  possibility  of  unbalancing.  The 
monocyclic  generator  can  be  loaded  to  its  fullest  extent 
with  either  lights  or  motors,  or  partly  with  lights  and  partly 
with  motors,  in  any  proportion. 

It  has  been  noted  that  the  regulation  of  polyphase  gen- 


MONOCYCLIC   SYSTEM. 


195 


erators  varies  with  the  inductive  character  of  the  load. 
The  monocyclic  generator,  when  designed  with  shunt  and 
series  excitation,  possesses  the  superior  advantage  of  auto- 
matically compounding  for  all  kinds  of  load.  The  power 


1) J lea^r-Wire 

f Feeder 


Pig.  ISO. 

wire  of  the  monocyclic  system  supplies  the  magnetizing 
current  to  the  motors,  which  current  is  returned  over  the 
main  wires,  adding  to  the  magnitude  of  the  current  in 
one  lead,  and  reducing  it  in  the  other.  The  commutating 
device  is  placed  in  the  main  carrying  the  largest  current. 
As  the  increase  over  the  normal  depends  on  the  motors,  — 


196       POLYPHASE  APPARATUS   AND   SYSTEMS. 

the  inductive  load,  —  the  greater  the  inductive  character 
of  the  load,  the  larger  will  be  the  series-exciting  current. 
In  this  way  the  monocyclic  machine  can  be  made  to  give 
perfect  compounding,  on  either  inductive  or  non-inductive 
loads. 

Generator  Armature  Connections.  —  The  connections  and 
detail  of  the  monocyclic  generator  armature  are  shown  in 
Fig.  1 39-  The  armature  coils  are  made  up  of  a  single-phase 
main  winding,  similar  to  the  ordinary  armature  winding  of  a 
single-phase  alternator.  Midway  between  the  main  slots  of 
the  armature  is  a  set  of  smaller  slots,  containing  the  auxili- 
ary winding  of  the  same  cross-section  as  the  main  winding, 
but  of  only  one  quarter  the  number  of  turns.  One  end 

of  the  teaser  coil  is 
connected  to  the  mid- 
die  of  the  main  coil, 
and  the  other  to  a 

2080  * 

Fig.  14O.  third  collector  ring. 

In  this  teaser  coil,  an 

E.M.F.,  in  quadrature  with  that  of  the  main  coil,  is  estab- 
lished, which  is  made  use  of  for  supplying  magnetizing 
current  for  the  operation  of  alternating-current  motors. 

When  the  generator  is  wound  for  2,080  volts,  the  teaser 
coil  has  one-quarter  the  number  of  turns,  and  gives  an 
E.M.F.  of  520  volts.  The  E.M.F.  between  the  terminals 
of  the  main  coil  and  the  free  end  of  the  teaser,  is  the  result- 
ant of  the  E.M.F.  in  the  two  coils,  and  is  shown  in  magni- 
tude and  direction  by  Fig.  140.  The  teaser  may  be  wound 
to  have  86  per  cent  of  the  main  turns,  instead  of  25  per 
cent ;  in  which  case  the  electromotive  forces  of  the  three 
terminals  are  equal,  and  we  have  a  three-phase  relationship. 

When  the  single-phase  circuit  is  loaded,  the  potential  be* 


MONOCYCLIC   SYSTEM. 


197 


tween  the  mains  does  not  bear  the  same  phase  relationship 
to  the  teaser  terminal  that  it  did  on  open  circuit.  The  cur- 
rent lags  behind  the  impressed  volts,  due  to  the  self-induc- 
tion. The  triangle  of  the  terminal  electromotive  forces  is 
distorted,  so  that,  if  the  main  potential  is  2,080  volts,  that 
between  the  teaser  and  one  main  may  be  1,320  volts,  and 
the  other  800  volts  (Fig.  141).  Loading,  now,  the  teaser 
wire,  produces  a  current  lag,  and  shifts  its  potential  so  that 


80O 


B 


Fig.  14L 


"60 


Fig.  142. 


the  triangle  of  E.M.F.  regains  its  normal  shape,  and  the 
electromotive  forces  their  magnitude  and  normal  relation- 
ship (Fig.  142). 

The  wiring  for  monocyclic  circuits,  when  lights  only  are 
supplied,  is  the  same  as  for  single-phase  circuits. 

Systems  of  Distribution.  —  In  Fig.  143  is  shown  a  dia- 
gram of  a  monocyclic  system  of  light  and  power  distri- 
bution. 

The  generator,  A ,  sends  power  over  the  main  wires,  a 


198       POLYPHASE  APPARATUS   AND   SYSTEMS. 

and  b,  to  transformer,  T,  operating  lights  and  a  small 
single-phase  fan  motor  from  its  secondaries. 

From  the  same  generator  issues  the  teaser,  or  power 
wire,  of  small  cross-section,  shown  in  dotted  lines,  c,  which 
is  carried  to  the  pairs  of  transformers,  D  and  E,  supplying 
motors. 

In  D  is  shown  the  arrangement  of  transformers  suitable 


Induction 
Motor 


System  O/  Secondary  Mains. 


Fig.  143. 


for  the  operation  of  standard  alternating-current  induction 
motors  from  their  secondaries.  The  two  transformers  are 
of  equal  size  and  of  one-half  the  main-line  voltages,  and  are 
connected  with  their  primaries  between  teaser  and  main 
wires,  while  one  of  their  secondaries  is  reversed  with 
regard  to  the  primary,  and  thereby  establishes,  in  the  sec- 
ondary circuit,  a  relation  of  electromotive  forces  suitable 
for  the  operation  of  the  motor. 


MONOCYCLIC  SYSTEM.  199 

In  E  an  arrangement  is  shown,  whereby  lights  and 
motors,  or,  in  short,  a  whole  three-wire  network,  is  operated 
from  the  transformer  secondaries. 

The  large  or  main  transformer  is  connected  between 
the  main  lines,  a  and  b,  and  is  of  a  size  sufficient  to  supply 
the  total  capacity  of  the  secondary  network.  An  addi- 
tional or  teaser  transformer,  of  one-quarter  the  primary 
main  voltage,  and  of  very  small  size  only,  is  connected  by 
one  terminal  to  the  centre  of  the  main  transformer  coils, 
while  the  other  terminal  connects  with  the  teaser  wire,  c, 
in  the  primary,  and  the  motor  wire  in  the  secondary.  This 
transformer  connection  is  analogous  to  the  connection  of 
main  and  teaser  coil  in  the  generator. 

Supplied  in  this  way,  a  secondary  network  on  the  mono- 
cyclic  system  consists  of  four  conductors,  —  two  main 
conductors,  the  lightning  neutral,  and  the  power  neutral  or 
balance  wire. 

Such  a  secondary  network  can  be  operated  in  the  same 
way  as  a  continuous-current  three-wire  system,  and  offers 
the  essential  advantage  oif  saving  the  excessfve"amount  of 
copper  in  the  long  feeders,  by  being  applied  from  high- 
potential  lines  through  transformers.  An  unbalancing 
due  to  the  motors  is  not  possible,  and  motors  operated  on 
this  system  do  not  affect  the  lights,  except  in  so  far  as  the 
ohmic  drop  in  the  mains  is  concerned. 

Arc  lights  can  be  operated  very  satisfactorily  from  the 
monocyclic  system,  and  are  supplied  either  by  compensa- 
tors from  the  secondary  circuits,  or  from  the  primary  cir- 
cuits by  transformers,  as  shown. 

Series  incandescent  lights  can  be  used  for  street  lighting, 
and  are  directly  supplied  from  the  primary  main  lines. 
Where  a  district  has  to  be  supplied,  which  is  too  far  dis- 


200   POLYPHASE  APPARATUS  AND  SYSTEMS. 

tant  to  be  reached  directly  by  the  primary  or  generator 
voltage,  step-up  and  step-down  transformers  may  be  used. 
Transformer  Connections — The  various  methods  of  con- 
necting transformers  to  monocyclic  circuits,  and  the  result- 


._ij_ 


U     i 


Teazer  Wir, 


Fig.  144. 

ant  voltages,  are  shown  in  detail  in  Figs.  144  to  146.  It 
will  be  noticed  the  teaser  wire  is  necessary  only  where 
motors  are  used,  the  lights  being  connected  on  the  single- 
phase  system.  Fig.  144  shows  the  detailed  connections 
and  standard  voltages  in  a  system  for  operating  lights  and 
motors  from  the  same  transformers. 

The  three-phase  relationship  for  operating  power  appa- 


MONOCYCLIC  SYSTEM. 


201 


ratus  may  be  obtained  by  the  transformer  connection,  as 
shown.  The  primaries  of  the  transformers,  which  are  of 
different  capacities,  are  connected  and  wound  to  produce 
the  exact  E.M.F.  relationship  of  the  generators.  The 
large  transformer  is  connected  across  the  main  circuit, 
while  the  supplementary  transformer  is  connected  to  the 
middle  of  the  large  transformer  and  to  the  teaser  wire. 
The  ratio  of  transformation  of  each  transformer  is  selected 


Generator 


Teazer  Wire 


2  Transformer* 


Fig.  145. 

so  that  the  secondary  E.M.F.  of  the  smaller  transformer 
is  about  82  per  cent  of  that  of  the  larger.  This  gives  a 
slightly  lop-sided  three-phase  relationship,  from  which 
motors  of  1 10  volts  and  lights  of  1 1 5  volts  can  be  operated. 
Of  course  an  exact  three-phase  relationship  can  be  ob- 
tained by  raising  the  E.M.F.  of  the  smaller  transformer  to 
86  per  cent.  The  smaller  transformer  should  be  about 
one-third  the  capacity  of  the  motor,  or,  if  a  number  of 


202   POLYPHASE  APPARATUS  AND  SYSTEMS. 

the  motors  be  used,  about   one-fourth  the  aggregate  ca- 
pacity. 

A  beautiful  illustration  of  the  resultant  three-phase  rela- 
tionship is  the  use  of  two  transformers  with  a  monocyclic 
generator,  the  secondary  of  one  being  reversed  (Fig.  145). 
The  diagram  of  E.M.F.  (Fig.  146)  shows  the  effect  of 


Fig.  146. 

reversing  the  secondary.  The  three  motor  wires  are  con- 
nected to  A,  C,  D\  the  difference  in  phase  being  nearly, 
though  not  quite,  60°.  The  secondary  circuits,  from  such 
an  arrangement,  may  be  considered  as  practically  the  same, 
and  have  all  the  advantages  of  a  straight  three-phase 
system. 

A  third  method  of  obtaining  the  proper  phase  relation- 
ship for  motor  work  is  shown  in  diagram  (Fig.  147).  In 
this  arrangement  only  one  transformer  is  required,  having 
half  the  capacity  required  for  the  other  methods  of  operat- 
ing motors.  The  primary  is  connected  between  one  of  the 
mains  and  the  teaser  wire.  The  secondary  coil  is  in  series 
with  the  primary,  and  has  the  same  number  of  turns ;  the 
ratio  of  transformation  being,  therefore,  I  to  I.  As  the 
primary  current  of  a  transformer  differs  from  the  second- 
ary current  1 80°  in  phase,  one  leg  of  the  circuit  is  naturally 
inverted,  changing  the  relation  of  the  phases  from  mono- 
cyclic  to  three-phase. 


MONOCYCLIC  SYSTEM. 


203 


This  arrangement  is  especially  suited  for  the  operation 
of  large  motors,  as  the  cost  of  transformers  is  reduced 
one-half.  The  voltage  of  the  motors  is  fixed  at  approxi- 
mately one-half  the  generator  voltage. 

The  most  common  and  convenient  connection  of  trans- 
formers, when  motors  alone  are  to  be  operated  from  a 
monocyclic  circuit,  is  that  shown  in  Fig.  145.  The  ratio 
of  transformation  of  the  transformer  here  shown  is  about 


—  c 

/WWv 

vAAAM 

/ 
It 

\ 

f-580-, 

<•—  580—  , 

<.  58 

3-V.  > 

Mo 

or 

Fig.  147. 

9  to  I .  In  the  operation  of  motors  from  1 ,040  volt  mono- 
cyclic  circuits,  transformers  of  the  ratio  of  4^  to  I  must  be 
used. 

Monocyclic  Motors.  —  The  three-phase  induction  motor  is 
the  most  suitable  for  use  on  monocyclic  circuits.  The  two- 
phase  motor  can  be  used  with  an  increase  in  the  number 
of  turns  of  the  teaser  winding,  but  at  the  expense  of  the 
output  of  the  generator,  considered  solely  as  a  single-phase 
machine.  The  performance  of  the  three-phase  motor,  con- 


2O4       POLYPHASE  APPARATUS   AND   SYSTEMS. 

nected  in  a  monocyclic  system,  in  regard  to  efficiency, 
torque,  and  power  factor,  is  essentially  the  same  as  on  a 
straight  three-phase  system.  The  flow  of  current  is  dif- 
ferent in  the  three  conductors,  the  teaser  wire  carrying 
mainly  the  magnetizing  current.  A  monocyclic  motor  of 
the  induction  type  can  be  used,  the  windings  of  which  are 
exact  reproductions  of  the  generator  windings,  —  i.e.,  are 
two  in  number;  one  having  25  per  cent  the  turns  of  the 
other,  and  connected  to  the  middle  of  the  large  coil.  Such 
a  motor  can  be  run  from  two  transformers  ;  or,  if  of  a  size 
permitting  it  to  be  wound  for  the  high  voltage  of  the  main 
circuits,  direct  from  the  mains.  While  the  monocyclic 
generator  can  be  fully  loaded  with  induction  motors,  which 
interchange  the  magnetizing  current  by  means  of  the  teaser, 
it  is  not  advisable  to  run  one  large  induction  motor  of  a 
size  approximating  that  of  the  generator.  In  special  cases, 
where  the  motor  need  not  have  a  large  starting  torque,  this 
arrangement  is  permissible. 

Synchronous  motors  on  a  monocyclic  system  need  not 
be  operated  from  reversed  transformers,  but  can  be  run 
direct  from  the  generators,  provided  they  are  identical  with 
them.  They  have  little  starting-torque,  and  require  an  ex- 
traneous source  of  power  to  bring  them  to  synchronism. 

Measurement  of  Power.  —  The  power  supplied  to  lights 
and  other  single-phase  current-consuming  devices,  is  meas- 
ured by  the  standard  forms  of  wattmeters.  On  account  of 
the  uncertain  flow  of  current  in  the  motor  connections, 
special  connections  are  necessary.  Fig.  148  shows  a  re- 
cording wattmeter  connected  to  measure  the  power  deliv- 
ered to  motors.  One  of  the  field  coils,  D,  is  connected  in 
the  common  return  B\  the  other  coil,  Ey  in  the  main  A,  or 
perhaps  in  the  main  C.  If  the  motor  is  loaded  and  the 


MONOCYCLIC  SYSTEM. 


205 


meter  speeds  up,  the  connections,  as  shown,  are  right.  If 
the  meter  speed  diminishes  with  increasing  motor  load,  the 
field  coil  E  should  be  connected  in  the  main  C.  This 
meter  will  be  found  to  give  fairly  accurate  results.  At 
high  loads  the  reading  will  be  found  slightly  high,  but  not 
sufficiently  to  be  commercially  objectionable. 


Shiunb 


Fig.  148. 

In  cases  where  great  accuracy  is  required,  two  meters  can 
be  used,  measuring  individually  the  output  of  the  two  trans- 
formers. Fig.  149  shows  the  method  of  measuring  the 
entire  output  of  a  monocyclic  generator.  Each  of  the  field 
coils,  a  and  b,  of  wattmeter  A,  are  connected  in  each  of 
the  mains.  The  other  pair  of  coils,  c  and  d,  of  wattmeter 


206   POLYPHASE  APPARATUS  AND  SYSTEMS. 

B,  are  in  series,  and  connected  in  the  teaser  wire.  .  To 
obtain  a  safe  voltage,  two  transformers  are  needed,  con- 
nected so  as  to  reproduce  the  phase  relationship  of  the 


Pig.  149. 

generator  windings.  Both  armatures  are  connected  to  a 
single  shaft,  rotation  being  due  to  the  resultant  action  of 
the  two  fields. 


CHOICE   OF   FREQUENCY.  207 


CHAPTER    XII. 
CHOICE  OF  FREQUENCY. 

High  Frequencies. — In  designing  a  plant  for  the  distri- 
bution of  light  and  power  by  polyphase  currents,  one  of  the 
first  considerations  that  presents  itself,  is  whether  the  ap- 
paratus shall  be  of  high  or  low  frequency.  By  high  fre- 
quency is  generally  understood  to  mean  one  of  over  60 
cycles  per  second,  or  7,200  alternations  per  minute.  Sixty 
cycles  and  less  are  considered  low  frequencies.  Until 
quite  recently,  the  frequencies  generally  employed  in  the 
United  States  were  125  and  133  cycles,  or  15,000  and 
16,000  alternations.  Abroad,  the  commercial  frequencies 
were  somewhat  lower,  varying  from  80  to  100  cycles.  The 
adherence  to  a  high  frequency  in  this  country  for  over  ten 
years  has  resulted  in  an  investment  of  millions  of  dollars  in 
this  particular  type  of  apparatus,  and  has  made  the  intro- 
duction of  new  types  of  lower  frequency  into  old  and  exist- 
ing central  stations  extremely  difficult,  even  when  evident 
economy  and  advantage  have  been  shown  to  follow  upon 
such  introduction. 

The  tendency  of  modern  alternating-current  practice  is 
in  the  direction  of  low  frequencies,  and  in  the  organization 
of  a  new  plant,  the  problem,  in  nine  cases  out  of  ten,  is 
confined  to  the  selection  of  a  frequency  of  60  cycles  or 
under. 

There  are  frequently  strong  reasons  for  retaining  or 


208       POLYPHASE   APPARATUS    AND    SYSTEMS. 

adopting  125  or  133  cycles.  One  of  these  has  been  men- 
tioned above.  The  change  from  125  cycles  to  a  lower 
frequency  necessitates  a  complete  revamping  of  the  instal- 
lation, and,  with  the  exception  of  the  small  sizes,  the  trans- 
formers must  be  replaced.  Again,  when  a  low  first-cost 
of  a  plant  is  considered  of  more  importance  than  a  possible 
ultimate  saving  of  operating  expenses,  and  a  more  satisfac- 
tory service,  a  high  frequency  will  be  used.  The  gener- 
ators are  cheaper,  as  they  run  at  a  higher  speed.  The 
transformers  are  also  smaller  and  cheaper. 

One  of  the  drawbacks  to  the  use  of  high  frequencies, 
especially  in  the  transmission  of  power  over  lines  of  consid- 
erable length,  is  the  drop  of  voltage  due  to  the  reactance  of 
the  line,  which  increases  with  the  frequency.  For  illustra- 
tion:  the  reactance  of  1,000  feet  of  No.  i  wire,  at  25 
cycles,  is  .0486  ohms,  and  at  125  cycles,  .243  ohms.* 

By  reducing  the  frequency  from  125  cycles  to  25  cycles, 
in  the  above  case,  the  voltage  drop,  due  to  the  reactance 
and  resistance,  is  reduced  almost  one-half.  With  heavier 
conductors  and  higher  frequencies,  the  difference  is  still 
more  noticeable.  The  effect  of  frequency  on  the  volt- 
age drop  in  transmission  lines  is  treated  at  further  length 
in  Chapter  XIV.  In  lighting  plants  employing  large 
conductors,  on  account  of  the  varying  power-factors  due 
to  changing  character  of  load,  the  irregularity  in  volt- 
ages at  high  frequencies  may  become  quite  marked.  As 
we  have  seen,  this  voltage  drop  is  not  all  energy  loss,  this 
loss  being  only  proportional  to  the  energy  component  of 
the  total  drop. 

Other  disadvantages  in  the  use  of  high  frequencies  are 
the  speed  at  which  both  generators  and  motors  must  run 

*  See  Table  of  Line  Constants  for  Power  Transmission,  page  224. 


CHOICE   OF    FREQUENCY.  209 

in  order  not  to  unduly  increase  the  number  of  poles,  and 
the  difficulty  in  connection  with  engine  regulation,  when  a 
number  of  generators  are  direct  driven,  and  operated  in 
parallel.  High-frequency  as  well  as  low-frequency  induc- 
tion motors  operate  better  at  high  speeds,  but  these  are 
undesirable  from  both  mechanical  and  commercial  stand- 
points. On  the  other  hand,  high-frequency  induction 
motors  of  reduced  speeds  have,  as  a  rule,  low  power-fac- 
tors. The  high-frequency  induction  motor  was  introduced 
to  meet  the  demand  for  motors  of  small  power  on  high 
frequency  circuits.  With  the  system  on  which  it  is  at 
present  operated,  it  may  in  time  become  a  thing  of  the 
past. 

To  sum  up :  High  frequencies  permit  the  use  of  cheap 
generators  and  transformers,  and,  in  addition,  the  simple 
and  satisfactory  operation  of  incandescent  and  arc  lamps 
and  synchronous  motors.  They  have  the  disadvantage  of 
increasing  the  voltage  drop  and  idle  currents  of  circuits, 
with  consequent  bad  regulation  and  heating  of  the  genera- 
tor at  light  loads,  of  not  permitting  the  parallel  operation 
of  direct-connected  machines  of  low  speed,  and  the  further 
disadvantage,  that  induction  motors  must  either  run  at 
excessive  speeds,  or  with  poor  power-factors.  Synchro- 
nous motors  will  not  start  with  the  same  vigor  as  on  lower 
frequencies. 

Low  Frequencies Up  to  the  present  time  no  arc  lamp 

has  been  made  that  will  operate  satisfactorily  on  fre- 
quencies lower  than  40  cycles.  At  this  frequency  the 
interruptions  of  the  arc  are  plainly  visible  to  the  eye. 
Incandescent  lamps  cannot  be  used  to  advantage  on  fre- 
quencies less  than  30  cycles.  Low-voltage  incandescent 
lamps  show  no  flicker ;  but  the  effect  of  fatiguing  the 


210   POLYPHASE  APPARATUS  AND  SYSTEMS. 

eye  is  quite  noticeable  at  25  cycles,  and  perceptible  after 
a  while  at  30  cycles.  The  current  reversals  are  easily 
distinguished  in  high  voltage  lamps  at  25  cycles  and 
under. 

Transformers  are  somewhat  bulkier,  more  expensive,  and 
slightly  less  efficient  at  low  frequencies.  Induction  motors, 
while  likewise  larger  and  more  expensive,  as  a  rule  can  be 
built  with  equal,  if  not  better,  power  factors,  and  at  con- 
venient and  commercial  speeds.  Rotary  converters  can  be 
successfully  designed  for  60  cycles.  The  speed  is  high, 
however,  and  the  best  mechanical  and  electrical  results  are 
obtained  at  frequencies  under  40  cycles.  The  largest  use 
of  miscellaneous  power  by  rotary  converters  is  at  Niagara, 
where  a  frequency  of  25  cycles  is  employed.  The  largest 
use  of  power  for  electric  railway  work  by  rotary  converters 
is  at  St.  Anthony  Falls,  Minneapolis ;  the  frequency  here 
being  approximately  35  cycles. 

The  Niagara  plant  is  essentially  a  power  plant.  The 
use  of  current  for  both  arc  and  incandescent  lighting  is  of 
no  great  importance.  The  power,  electrically  generated 
on  a  scale  never  before  attempted,  is  used  locally  in  a  great 
variety  of  processes,  and  is  delivered  in  a  form  most  suit- 
able for  its  diverse  uses.  Power  by  the  direct-current 
system,  while  convenient  for  some  particular  operations, 
would  not  answer  equally  well  all  requirements  at  Niagara, 
and  would  be  unsuitable  for  long-distance  transmission. 
A  high-frequency  system  would  restrict  the  use  of  motors 
and  rotary  converters,  and  the  transmission  of  power  over 
very  long  distances.  Sixty  and  40  cycles,  however,  permit- 
ting the  general  use  of  lighting  apparatus,  do  not  give 
the  best  results  with  rotary  converters  of  large  output. 
The  operation  of  25  cycle  rotary  converters,  on  the  scale 


CHOICE   OF   FREQUENCY.  211 

employed  at  Niagara,  shows  that,  for  the  purely  power 
conditions  there  existing,  this  frequency  was  wisely  chosen. 

A  frequency  of  25  cycles  is  also  used  by  the  Brooklyn 
Edison  Illuminating  Company  in  the  recent  extension  of 
their  plant.  Four  thousand  H.P.  are  transmitted  within 
an  area  covering  75  miles,  to  various  substations,  where  25 
cycle  rotary  converters  are  stationed.  These  deliver  1 1 5 
volt  direct-current  into  Edison  three-wire  mains.  The 
Chicago  Edison  Company  use  a  somewhat  similar  system 
of  distribution  and  the  same  frequency. 

For  the  general  conditions  of  a  power  plant,  supplying 
alternating  current  for  induction  motors  and  lighting,  and 
making  a  specialty  of  furnishing  direct  current  on  a  large 
scale,  at  some  distance  from  the  generating  plant,  a  fre- 
quency of  35  to  40  cycles  will  be  found  suitable. 

The  frequency  of  60  cycles,  or  7,200  alternations  per 
minute,  has  come  into  extensive  use.  It  has  the  advan- 
tage of  considerably  reducing  line  reactance  and  the  idle 
currents  present  in  lighting  systems  of  higher  frequencies. 
It  is  adapted  for  the  most  economical  results  in  a  general 
distributing  system  of  lights  and  motors.  On  account  of 
the  good  regulation  possible  with  this  frequency,  the  high- 
est economy  lamps  can  be  used.  Sixty  cycle  motors  are 
excellent  in  respect  to  efficiency  and  power  factor,  and  run 
at  commercial  speeds.  Both  motors  and  transformers  are 
reasonable  in  cost. 

When  the  generating  units  are  direct-connected  to 
engines  of  extremely  slow  speed  and  operated  in  parallel, 
a  frequency  of  60  cycles  will  be  found  to  be  not  desirable. 
As  explained  in  Chapter  III.,  the  permissible  variation  in 
rotative  speed  is  not  so  great  as  with  lower-frequency  gen- 
erating units, 


212       POLYPHASE   APPARATUS   AND    SYSTEMS. 

Choice  of  Frequency.  —  It  is  impossible  to  make  general 
applications  of  the  foregoing  remarks.  Each  particular 
case  must  be  studied  in  the  light  of  its  special  conditions, 
before  an  intelligent  decision  can  be  made  as  to  the  proper 
frequency  to  employ.  At  the  risk  of  repetition,  the  fol- 
lowing general  recommendations  are  suggested  as  embody- 
ing the  latest  and  standard  practice : 

For  local  lighting  systems  with  incidental  demand  for 
power  in  small  units,  where  old  transformers  have  to  be 
retained,  and  where  a  cheap  plant  is  of  first  consideration, 
a  high  frequency  may  be  used,  but  should  be  discouraged 
as  much  as  possible. 

For  general  transmission  and  distribution  for  lighting 
and  power  purposes,  conditions  which  accompany  the  ma- 
jority of  alternating-current  propositions,  a  standard  fre- 
quency of  60  or  66  cycles  should  be  used. 

In  power  and  lighting  plants,  —  where  arc  lighting  is  of 
secondary  consideration,  —  supplying  current  to  induction 
motors,  as  in  mill  work,  and  to  rotary  converters,  as  in 
long-distance  railway-transmission  work,  where  the  gen- 
erators are  direct  driven  by  engines,  and  finally,  for 
very  long  transmissions  of  power,  a  frequency  of  40 
cycles,  or  thereabouts,  may  be  used.  This  is  a  good,  all- 
round  frequency,  and  is  coming  into  more  general  use  in 
this  country.  It  is  the  frequency  generally  employed 
abroad. 

For  exclusively  power  plants,  where  lighting  is  of  no 
importance  whatsoever,  and  where  rotary  converters  and 
motors  of  large  size  or  slow  speed  are  to  be  supplied,  a 
frequency  of  25  to  30  cycles  may  be  used. 

Notwithstanding  the  opportunity  for  the  careful  exercise 
of  judgment  in  selecting  a  proper  frequency,  almost  equally 


CHOICE   OF   FREQUENCY.  213 

good  results  can  have  been  obtained  with  widely  different 
frequencies.  As  an  illustration,  the  Brooklyn  Edison 
Company  have  adopted  25  cycles  for  their  power  and 
rotary  converter  work.  The  Boston  Electric  Light  Com- 
pany obtain  practically  the  same  results,  using  a  frequency 
of  60  cycles. 


214       POLYPHASE   APPARATUS   AND    SYSTEMS. 


CHAPTER    XIII. 

RELATIVE    WEIGHTS  OF   COPPER   FOR   VARIOUS 
SYSTEMS. 

As  the  transmission  and  distribution  of  power  often 
involves  a  large  outlay  for  copper  conductors,  it  is  most 
important  to  ascertain  what  system  and  what  combination 
of  conductors  will  give  the  most  economical  results.  In 
making  any  comparison  between  the  copper  efficiencies  of 
the  various  systems,  the  proper  basis  of  comparison  is 
equality  of  voltage. 

The  amount  of  copper  required  for  transmitting  a  given 
power  at  a  fixed  percentage  loss  is  found  by  the  rule  that 
the  weight  of  copper  varies  inversely  as  the  square  of  the 
voltage. 

The  voltage  of  an  alternating  circuit,  as  measured  by 
the  ordinary  commercial  instruments,  —  i.e.,  the  effective 
voltage,  —  is  about  40  per  cent  less  than  the  maximum 
value  of  the  E.M.F .  wave.  It  is  this  maximum  value  that 
must  be  considered  in  determining  the  break-down  point 
of  insulation  and  the  highest  voltage  that  can  be  used 
commercially,  as  in  the  long-distance  transmission  of 
power.  On  the  other  hand,  when  the  maximum  voltage 
of  a  circuit  is  within  the  limit  of  safe  insulation  strain,  the 
effective  voltage  carries  no  limitation  with  it. 

The  comparison,  then,  of  the  various  systems,  to  deter- 
mine the  most  economical  method  of  transmission,  will  be 


RELATIVE   WEIGHTS   OF   COPPER.  215 

either  on  the  basis  of  maximum  potential,  as  in  the  case  of 
long  transmission  lines,  or  on  the  basis  of  effective  or  min- 
imum potential,  as  in  the  case  of  low-potential  distributions 
by  secondary  mains. 

Figs.  150  to  156  show  the  standard  systems  of  alternat- 
ing-current distribution  and  the  various  combinations  of 
conductors  in  general  use.  The  name  of  each  system  is 
given,  and  also  the  relative  amount  of  copper  required. 

The  percentual  amount  of  copper  required  by  the  single- 
phase  system,  which  is  here  taken  as  the  standard  of  com- 
parison for  the  other  systems  and  combinations,  is  illus- 
trated by  diagram  (Fig.  150).  The  single-phase  three- 
wire  system  is  shown  in  Fig.  151.  If  the  voltage  of  the 
two-wire  system  is  *?,  the  potential  between  the  two  outside 
wires  is  2e.  Applying  the  rule  that  the  amount  of  copper 
is  inversely  as  the  square  of  the  voltage,  only  £  the  copper 
would  be  needed,  if  the  neutral  should  have  no  cross-sec- 
tion, or  the  return  conductor  be  dispensed  with,  as  might 
be  done  in  the  case  of  a  perfect  balance.  If  the  neutral  is 
given  a  cross-section  equal  to  one  of  the  outside  wires,  the 
total  copper  in  the  three-wire  single-phase  system  is  37.5 
per  cent  that  of  the  two-wire  single-phase  system.  With 
a  neutral  -J-  and  \  the  cross-section  of  the  outside  wires,  the 
total  copper  is  31.25  per  cent  and  29.15  per  cent  respec- 
tively of  our  standard  system.  In  a  four-wire  system  the 
voltage  between  outside  wires  is  3^,  and,  under  perfect  bal- 
ance, \  the  amount  of  copper  would  be  required.  When 
the  neutral  and  outside  wires  are  of  equal  size,  the  copper 
must  be  increased  to  22.2  per  cent.  In  like  manner  the 
copper  in  the  five-wire  system,  with  neutrals  of  full  cross- 
section,  is  1 5 .62  per  cent,  and  the  same  system,  with  neu- 
trals of  \  the  area  of  the  neutral  wires,  requiring  only  10.93 


2l6       POLYPHASE   APPARATUS   AND   SYSTEMS. 


SYSTEM 

W//?/M?  CONNECTIONS                   PER  CENT. 
COPPER 

Single  Phase  \ 
2  Wire         f 

100. 

Fig.  ISO 

Single  Phase  > 

07  e 

3  Wire        | 

T«/o  Phase         IV 

*•*  U> 

Fig.  151 

1 

1*1 

100. 

TJ 

—  1 

jrt 

DIAGRAM 


Three  Phase 
3  Wire 


Three  Phase 
4  Wire 


Fig.  1£54 


Fig.  1£55 


75. 


33.3 


(  100. 
i  125. 
(  150. 


I 


L 


A 


per  cent  of  the  copper  of  the  simple  alternating  circuit. 
These  results  are  the  same  whether  the  comparison  is 
made  on  the  basis  of  maximum  potential,  or  on  the  basis  of 
effective  or  minimum  potential 

In  the  three-phase  system,  the  copper  required  for  cer- 


RELATIVE   WEIGHTS   OF   COPPER.  217 

tain  given  conditions  is  75  per  cent  of  the  copper  used  in 
the  single-phase  system.  The  comparison  between  poly- 
phase systems  can  best  be  made  by  resolving  each  into  as 
many  single-phase  systems  as  it  has  phases.  The  three- 
phase  system  consists  of  three  single  circuits  with  a  com- 
mon ground,  or,  what  is  the  same,  with  no  return  ;  for  the 
total  current  to  and  from  the  centre  is  zero.  If  the  A  or 
line  voltage  is  e  (Fig.  1 54),  the  pressure  or  volts  between 

a 
any  wire  and  the  juncture  is  — —  .     The  two-phase  system, 

V3 
having  a  line  voltage  e,  can  also  be  connected  into  two 

/? 

single  circuits  of  voltage  -  (Fig.  152).     As  the  weight  of 

copper  in  each  system  is  inversely  as  the  square  of  the 
voltage,  we  have : 

(2\2       /-\A\2 
_J    :  lji.£l.  =4  ;  3  —  or  the  relative  amounts  of  copper, 

for  the  two-phase  and  the  three-phase  systems,  are  100 
per  cent  and  75  per  cent. 

The  two-phase  four-wire  system,  consisting  of  two  single- 
phase  systems,  is  placed,  in  respect  to  the  amount  of  cop- 
per required  for  equal  conditions,  in  the  same  position  as 
the  single-phase  system. 

Fig.  153  illustrates  the  two-phase  three-wire  distribu- 
tion, two  of  the  wires  of  the  four-wire  system  being  replaced 
by  one  of  full  cross-section.  The  voltage  between  the 
two  outside  conductors  is  now  raised  to  ^/  2e=  1.412  e,  e 
being  the  potential  between  the  conductors  of  either  phase. 
The  amount  of  copper  required,  when  compared  with  the 
single-phase  system,  will  differ  considerably  according  as 
the  comparison  is  based  on  the  highest  voltage  permissible 
for  any  given  distribution,  or  on  the  minimum  voltage  for 


2l8       POLYPHASE  APPARATUS   AND   SYSTEMS. 

low-tension  service.  If  e  is  the  maximum  voltage,  that 
can  be  used  on  account  of  the  insulation  strain,  or  for  any 
other  reason,  the  pressure  between  the  other  conductors 
of  the  two-phase  three-wire  system  must  be  reduced  to 

e 
—  o     The  weight  of  copper  required  under  this  condition 

V2 

is  145.7  Per  cent  °f  the  single-phase  copper.  If  the  limit- 
ing conditions  of  voltage  do  not  exist,  a  comparison  of  the 
relative  weights  of  copper  can  be  made  with  the  effective 
voltage  of  either  phase  as  a  basis,  —  i.e.,  on  a  basis  of  the 
minimum  voltage.  In  this  case  we  find  a  relative  saving 
over  the  single-phase  circuit  of  about  27  per  cent,  the 
actual  amount  of  copper  being  72.9  per  cent  of  the  single- 
phase  conductors. 

Fig.  1 5  5  shows  the  connections  of  the  three-phase  four- 
wire  system.  When  the  fourth  wire,  or  neutral,  is  of  full 
cross-section,  the  copper  required  is  331  per  cent  of  the 
single-phase  system.  By  making  the  neutral  one-half  the 
cross-section  of  the  main  conductors,  the  copper  weight  is 
reduced  to  29.17  per  cent.  This  arrangement  is  only  used 
for  secondary  systems  of  distribution,  as  described  before. 
The  comparison  with  any  other  system  is,  therefore,  made 
only  on  a  basis  of  equality  between  phases  of  minimum 
voltage. 

The  monocyclic  system  (Fig.  156)  is  treated  as  a  single- 
phase  system  in  the  calculation  of  its  lighting  circuits. 
When  motors  are  connected  to  the  circuit,  the  single-phase 
copper  is  increased  proportionally  to  the  motor  load,  and 
by  the  teaser  wire.  The  rule  governing  the  size  of  the 
teaser  wire  is,  that  its  cross-section  should  bear  the  same 
relation  to  that  of  the  main  wires  that  the  motor  load  does 
to  the  total  load. 


RELATIVE   WEIGHTS   OF   COPPER. 


219 


If  the  teaser  is  made  of  a  cross-section  equal  to  one  of 
the  main  conductors,  the  total  weight  of  copper  is  150  per 
cent  of  that  in  a  single-phase  circuit  of  equal  voltage  and 
power.  If  the  load  is  equally  divided  between  motors  and 
lights,  the  teaser  has  a  cross-section  of  one-half  the  main 
conductors,  and  the  total  copper  is  125  per  cent  of  the 
single-phase  copper.  The  main  circuit  can  be  connected 
as  a  three-,  four-,  or  five-wire  system.  The  amount  of  cop- 
per required  is  found  by  adding  the  proportionate  weight 
of  the  teaser  wire.  In  this  way  a  three-wire  monocyclic 
circuit,  neutral  one-half  cross-section,  loaded  one-half  with 
lights,  one-half  with  motors,  will  require  39  per  cent  of  the 
copper  of  the  single-phase  system. 

The  following  Tables  are  compiled  from  data  in  Mr. 
Steinmetz's  valuable  work,  "  Alternating-Current  Phenom- 
ena." The  first  Table  gives  the  relative  copper  efficiencies 
of  various  systems,  when  the  comparison  is  on  the  basis  of 
equality  of  minimum  difference  of  potential.  The  second 
gives  the  relative  weights,  when  the  comparison  is  based 
on  the  equality  of  the  maximum  potential  difference  in 
the  system. 

Amount  of  copper  required  for  transmission  at  a  given  loss,  based 
on  minimum  potential. 


SYSTEM. 

No.  OF  WIRES. 

PER  CENT 
COPPER. 

Single-phase         .     .                   .... 

2 

IOO 

Single-phase    
Two-phase,  common  return  .         .    .    . 
Two-phase   ...                       . 

3 

3 

4 

37-5 
72.9 

IOO 

Three-phase     

7 

7C. 

Three-phase,  neutral  full  section  .    .    . 
Three-phase,  neutral  one-half  section    . 

4 
4 

33-3 
29.17 

220       POLYPHASE  APPARATUS   AND   SYSTEMS. 


Amount  of  copper  required  for  transmission  at  a  given  loss,  based 
on  maximum  difference  of  potential. 


SYSTEM. 

No.  OF  WIRES. 

PER  CENT 

COPPER. 

Single-phase     

2 

IOO. 

Two-phase,  with  common  return  .     .     . 

3 

145-7 

Two-phase 

IOO 

Three-phase     

3 

75- 

Direct  Current     

2 

5°- 

It  will  be  seen  that  the  direct-current  system  requires 
only  50  per  cent  of  the  copper  in  the  single-phase  system 
when  used  in  long-distance  transmission  of  power.  The 
advantage  is  not  so  evident,  however  ;  for,  as  Mr.  Stein- 
metz  has  pointed  out,  in  addition  to  the  electrostatic  stress, 
an  electrolytic  effect  is  set  up,  which  does  not  exist  to  the 
same  extent  in  alternating  currents.  The  difficulties  at- 
tending the  utilization  of  direct  current  of  high  tension, 
are  such  that,  with  the  exception  of  one  or  two  special 
and  isolated  cases,  its  employment  in  the  long  distance 
transmission  of  power  has  not  been  seriously  considered. 


CALCULATION   OF   TRANSMISSION   LINES.      221 


CHAPTER  XIV. 
CALCULATION   OF  TRANSMISSION    LINES. 

Line  Constants. — As  explained  in  Chapter  I.,  the  drop 
of  voltage  in  an  alternating-current  circuit  will  vary  with 
the  resistance  and  the  reactance  of  the  circuit,  and  with 
the  character  of  the  load.  In  the  table,  "  Line  Constants 
for  Power  Transmission,"  taken  from  a  publication  of  the 
General  Electric  Company,  the  relation  of  reactance  to 
resistance  is  shown  for  a  number  of  frequencies,  and  for 
the  sizes  of  conductors  ordinarily  used  in  power  transmis- 
sions, and  also  other  constants  of  transmission  circuits, 
such  as  capacity  inductance  and  charging  current.  The 
following  explanations  will  serve  to  make  the  table  clear : 

The  E.M.F.  consumed  by  resistance  r,  of  the  line,  is  =  Ir, 
and  in  phase  with  the  current  / 

The  E.M.F.  consumed  by  the  reactance,  S,  of  the  line,  is  = 
/S,  and  in  quadrature  with  the  current  / 

The  E.M,F.  consumed  in  the  line,  is  neither*  Ir  nor  IS,  but 
depends  upon  the  phase  relation  of  current  in  the  receiving 
circuit. 

The  loss  of  energy  in  the  line  is  =  /V,  hence  does  not  de- 
pend upon  the  reactance,  but  only  upon  the  resistance. 

Two  wires  in  parallel  have  the  same  resistance,  and  about 
half  the  reactance  (if  strung  on  separate  insulators  and  inter- 
mixed) of  a  single  wire  of  double  cross-section.  Thus  replacing 
one  No.  oooo  wire  by  two  No.  o  wires,  the  resistance,  weight 


222       POLYPHASE  APPARATUS   AND   SYSTEMS. 

of  copper,  etc.,  will  remain  the  same,  but  the  reactance  will  be 
reduced  practically  to  half,  so  where  lower  reactance  is  desired, 
the  use  of  several  conductors  strung  on  independent  insulators 
and  intermixed  is  advisable. 

The  values  given  for  Z,  (7,  i ,  and  S  are  calculated  for  sine- 
waves  of  current  and  E.M.F. 

This  table  will  be  found  most  convenient  for  determin- 
ing the  characteristics  of  transmission  circuits  when  the 
size  of  conductor  has  been  fixed. 

Let  us  take,  as  an  example,  a  case  where  it  is  required 
to  deliver,  by  the  three-phase  60  cycle  system,  2,000  H.P. 
at  the  secondary  terminals  of  the  step-down  transformers, 
over  a  circuit  1 1  miles  in  length.  It  is  further  assumed 
that  the  voltage  at  the  receiving  end  is  10,000,  and  the 
total  energy  loss  in  transmission  from  the  generator  ter- 
minals is  not  to  exceed  1 5  per  cent.  The  power  is  to  be 
used  for  a  mixed  system  of  lights  and  induction  motors, 
the  latter  forming  most  of  the  load.  The  power  factor  of 
the  system  at  the  receiving  end  will  be  approximately  85 
per  cent.  We  can  assume  that  — 

The  transformers  have  an  efficiency  of  97^  per  cent. 
The  copper  loss  in  each  being  i  per  cent. 
The  core  or  hysteresis  loss,  i^-  per  cent. 
The  reactance  can  be  taken  as  3^  per  cent. 
And  the  magnetizing  current  4  per  cent. 

The  voltage  between  any  branch  of  the  circuit  and  the 
common  centre  of  the  system  is 

10,000     „„ 

==—  =  O./7O. 

V3 
The  energy  delivered  by  each  branch  is 

K.W.  =  500  K.W. 


CALCULATION    OF   TRANSMISSION    LINES.      223 

elivered  by  e 
=  588  K.W. 


The  apparent  energy  delivered  by  each  branch  is 
500 


.85 

The   total    current    in    each    branch    is  -  -  =  102 

amperes. 

The  I.R.  drop  in  each  branch  is  10  per  cent  of  5,775  = 

577.5  volts. 

577-5 

The  total  resistance  R  =•  —     -  =  5 .66  ohms. 

1 02 

The  resistance  of  one  mile  is  -    -  =  .514  ohms,  which 

is  very  nearly  the  resistance  of  No.  o  wire.  Three  No.  o 
wires,  therefore,  will  carry  2,000  H.P.  a  distance  of  n 
miles  with  a  waste  of  energy  of  10  per  cent,  the  pressure 
at  the  receiving  end  being  10,000  volts  and  power  factor 
85  per  cent. 

By  referring  to  the  following  table  the  characteristics  of 
this  transmission  line  are  readily  obtained.  The  reactance 
of  eleven  miles  of  single  conductor  is  seen  to  be  6.62  ohms 
at  the  frequency  employed.  The  inductance,  or  what  is 
the  same  thing,  the  coefficient  of  self-induction  of  the  line, 
is  17.6  Millihenrys.  The  charging  current  of  each  line  for 
the  eleven  miles,  with  the  given  voltage  and  frequency,  is 
found  to  be  .4  of  an  ampere. 

It  is  interesting  to  know  what  the  impressed  or  genera- 
tor E.M.F.  and  the  distribution  of  current  will  be,  in  this 
case,  when  the  plant  is  fully  loaded.  For-  this  investiga- 
tion, the  entire  system  may  be  reduced  to  a  uniform  vol- 
tage, by  multiplying  the  voltages  by  the  various  ratios  of 
transformation,  thus  bringing  both  the  secondary  pressure 
at  the  step-down  transformers,  and  the  generator  pressure, 
to  the  line  voltage.  The  current  values  are,  of  course, 
inversely  changed.  The  power  factor  of  the  load,  hav- 
ing been  assumed  as  .85,  the  induction  factor  will  be 
Vl  -  (.85)*  =  =52- 


224   POLYPHASE  APPARATUS  AND  SYSTEMS. 


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CALCULATION   OF   TRANSMISSION    LINES.     22S 


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226   POLYPHASE  APPARATUS  AND  SYSTEMS. 


In  Chapter  I.,  it  has  been  shown  that  the  impressed 
E.M.F.  is  made  up  of  two  component  parts,  one  in  phase 
with  the  current  and  called  the  energy  component  of  the 
E.M.F.,  the  other  in  quadrature  with  the  current  and 
called  the  induction  component.  In  symbols  : 

Impressed  E.M.F. 

=  V2  (Energy  comp.)2  +  2  (Ind.  comp.)2 

To  obtain  the  total  E.M.F.,  it  is  necessary,  then,  to  calcu- 
late separately  all  the  energy  and  induction  components  of 
the  circuit,  and  obtain  a  combined  resultant. 

With  the  values  already  assumed,  and  consulting  the 
preceding  table,  we  obtain  the  following  results  : 


CIRCUIT. 

VOLTAGE. 

CURRENT 

AMPERES. 

ENERGY 
COMPO- 
NENT. 

IND.  COM- 
PONENT. 

Secondary  Circuit. 

Energy  Component,       .85  X  5,775, 
Induction  Component^  .52  X  5,775, 

4,909 

3,003 

Current, 

102 

Step-down  Transformers. 

Resistance  loss  =  I.R.  =  i%  of  5,775, 

58 

Reactance  loss  =  I.S.  =  3%%  of  5,775, 

202 

Hysteresis  loss  =  i%%  of  102, 

i-5 

4,967 

3>2°5 

103-5 

Line. 

Resistance  loss  =  I.R.  =  103.5  X  5.72, 

592 

Reactance  loss  —  I.S.  —  103.5  X  6.62, 

685 

J  (5,559)*  4-  (3,890)*  =  6,785=  volts  at 

terminals  of  step-up  transformers. 

5,559 

3,890 

103.5 

Step-up  Transformers. 

Resistance  loss  =  I.R.  =  i%  of  6,785, 

68 

Reactance  loss  =  I.S.  =  3^%  of  6,785, 

238 

Hysteresis  loss                =  i)J%of  103.5, 

1.5 

N/  (5,628)2  4-  (4,128)*  —  6,980  —  volts  at 

generator. 

5*627 

4,128 

105. 

CALCULATION   OF  TRANSMISSION  LINES.      227 

The  energy  E.M.F.  between  any  one  line  and  the  neutral 
at  the  generator  end  is  seen  to  be  5,627,  and  the  volts  con- 
sumed by  the  reactance  of  the  system,  4,128.  The  total 
volts  required  at  the  generator  terminals  are  found  to  be 
1 2 1  per  cent  of  the  voltage  at  the  secondaries  of  the  trans- 
formers, reduced  to  the  line  voltage,  —  i.e.,  with  10,000 
equivalent  volts  between  the  lines  at  the  transformer  sec- 
ondaries, the  pressure  at  the  generator  must  be  12,100 
volts.  The  current  delivered  by  the  generator  to  the  line 
is  105  amperes,  and  is  3  per  cent  more  than  the  current  in 
the  secondary  circuits.  The  effect  of  the  transformer  core 
losses  is  the  same  as  if  a  corresponding  current  was  con- 
sumed by  lamps  or  other  apparatus  connected  across  the 
mains.  The  volt-ampere  output  of  the  generator  is  125  per 
cent  of  the  apparent  watts  at  the  receiving  end.  The  power 
factor  of  the  entire  system  is  found  to  be  about  80  per  cent. 

Simple  Wiring  Formulas. —  A  simple  and  sufficiently 
accurate  determination  of  the  sizes  of  conductors,  voltage 
drop,  and  distribution  of  currents,  in  any  direct  or  alternat- 
ing-current system,  can  be  made  from  the  general  formula 
based  on  Ohm's  law,  modified  by  the  use  of  the  proper 
constants.  The  former  formula  and  constants  will  be  found 
especially  useful  and  convenient  for  this  calculation : 

D  x  W 

Area  of  conductor,  Circular  Mils  =  — —2  X  K 

P  X  E 

Volts  loss  in  lines  =  —         -  X  M 
100 

W 

Current  in  main  conductors  =  —  X  T 

H/ 

D  =  Distance  of  transmission  (one  way)  in  feet. 
W  =  Total  watts  delivered  to  consumer. 
P  =  Per  cent  loss  in  line  of  W. 

E  =  Voltage  between  main  conductors  at  receiving  or  con- 
sumer's end  of  circuit 


228       POLYPHASE   APPARATUS  AND    SYSTEMS. 


VALUES  OF  K. 

VALUES  of  T. 

SYSTEM. 

PER    CENT    POWER    FACTOR. 

PER    CZNT    POWER    FACTOR. 

IOO 

95 

90 

85 

80 

95 

90 

85 

80 

Single-phase  . 

2,  1  60 

2,400 

2,660 

3,000 

3,38o 

1.052 

I.  Ill 

1.172 

1.250 

Two-phase  (4- 

wire)  .     .     . 

1,  080 

I,2OO 

',330 

1,500 

1,690 

.526 

•555 

.588 

.625 

Three  -  phase 

(3-wire)   .     . 

1,  080 

I,2OO 

!,33° 

1,500 

1,690 

.607 

.642 

.679 

•725 

Values  of  the  constant,  K,  for  any  particular  power-factor 
are  obtained  by  dividing  2,160  by  the  square  of  that  power 
factor  for  single-phase,  and  by  twice  the  square  of  that 
power  factor  for  three-wire  three-phase  or  four-wire  two- 
phase.  The  resistance  of  line  wire  is  taken  as  10.8  ohms 
per  mil  foot. 

T  is  a  variable,  depending  on  the  system  and  nature  of 
the  load,  and  equal  to  I  for  continuous  current,  and  for 
alternating  current  with  100  per  cent  power-factor.  Its 
value  for  two-phase  and  three-phase  systems  is  .50  and  .58 
respectively,  with  100  per  cent  power-factor. 

M  is  a  variable,  depending  on  the  size  of  wire,  frequency, 
and  power  factor.  It  is  equal  to  i  for  continuous  cur- 
rent, and  for  alternating  current  with  100  per  cent  power- 
factor  and  sizes  of  wire  given  in  the  following  table  of 
wiring  constants. 

The  values  of  M,  as  given  in  the  table,  are  empirical. 
They  are  sufficiently  accurate  for  all  practical  purposes, 
provided  the  displacement  in  phase  between  current  and 
E.  M.  F.  at  the  receiving  end  is  not  very  much  greater  than 
that  at  the  generator  ;  in  other  words,  provided  that  react- 
ance of  the  line  is  not  excessively  large,  or  the  line  loss 
unusually  high.  For  example,  the  constants  should  not  be 


CALCULATION   OF  TRANSMISSION   LINES.     229 

applied  at  125  cycles  if  the  largest-size  conductors  were 
used,  and  the  loss  20  per  cent  or  more  of  the  power  deliv- 
ered. At  lower  frequencies,  however,  the  constants  are 
reasonably  correct,  even  under  such  extreme  conditions. 
They  represent  about  the  true  values  at  10  per  cent  line 
loss,  are  close  enough  at  all  losses  less  than  10  per  cent, 
and  often,  at  least  for  frequencies  up  to  40  cycles,  close 
enough  for  even  much  larger  losses. 

In  using  the  above  formulas  and  constants,  it  should  be 
particularly  observed  that  P  stands  for  the  per  cent  loss  in 
the  line  of  the  delivered  power,  and  not  for  the  per  cent 
loss  in  line  of  the  power  at  the  generator. 


VALUES   OF  M. 

No. 

WEIGHT 
OF  BARE 

30  CYCLES. 

60  CYCLES. 

125  CYCLES. 

OF 

AREA 

WIRE 

WIRE 

CIRCULAR 

PER 

. 

. 

. 

B.  & 

MILS 

0     &: 

H    • 

1/3           * 

i  bi 

fj-  H     * 

• 

O 

H    • 

. 

1,000   FT. 

z  ;.;(*• 

S.  G. 

POUNDS. 

|a* 

oO^ 

p-^* 

ij* 

nSd, 

ifi« 

P^ 

H  jj0* 

EJ£ 

P* 

II* 

h4        ^ 

**  2  oo 

^      08 

J     £ 

§00" 

^ 

J         o^ 

Soo>||C*       <^ 

•* 

< 

0000 

211,600 

640.73 

1.26 

1.27 

1.24 

1.64 

1.85 

1.85 

2.44 

3.06 

3-14 

ooo 

167,805 

508.12 

I.  2O 

1.17 

1.14 

1.49 

1.63 

1.62 

2.15 

2.62 

2.67 

00 

133,079 

402.97 

1.1$ 

1.  08 

1.05 

1.39 

1.46 

1.42 

1.92 

2.25 

2.29 

o 

I05,592 

3!9-74 

I.IO 

I.OO 

I.OO 

1.30 

1.32 

1.28 

i.73 

1.96 

1.99 

I 

83,694 

253-43 

1.06 

I.OO 

I.OO 

1.23 

1.  21 

1.16 

1.57 

1.74 

i.73 

2 

66,373 

200.98 

1.03 

I.OO 

I.OO 

1.16 

I.  II 

i.  06 

1.44 

1.54 

'•S3 

3 

52,633 

I59-38 

i.  02 

I.OO 

I.OO 

i.  ii 

1.04 

I.OO 

1.35 

1.38 

1-38 

4 

41,742 

126.40 

I.OO 

I.OO 

I.OO 

1.07 

I.OO 

I.OO 

1.26 

1.26 

1.22 

5 

33,102 

100.23 

I.OO 

I.OO 

I.OO 

1.04 

I.OO 

I.OO 

1.19 

1.16 

I.  II 

6 

26,250 

79-49 

I.OO 

I.OO 

I.OO 

1.02 

I.OO 

I.OO 

1.14 

i.  08 

1.03 

7 

20,816 

63-03 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

1.09 

I.OI 

I.OO 

8 

16,509 

49.99 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

I.OO 

i.  06 

I.OO 

I.OO 

230       POLYPHASE  APPARATUS   AND   SYSTEMS. 

» 
APPLICATION  OP  FORMULAS. 

SINGLE-PHASE    SYSTEM. 125    CYCLES. 

EXAMPLE  :  750  52-volt  lamps,  consuming  a  total  of 
45,000  watts.  Ratio  of  transformation  20  to  i.  Distance 
to  generator,  2,500  feet.  Loss  in  secondary  wiring,  2  volts. 
Voltage  drops  in  transformers,  2  per  cent.  Energy  loss  in 
line,  5  per  cent  of  delivered  power.  Efficiency  of  trans- 
formers, 97^  per  cent. 


Watts  at  transformer  primaries 

45,000 


=  47>IO°- 


2     =4.4  per  cent. 


.98  X  -97i 
Volts  at  transformer  primaries 

=  (52    +  2)    X    20    X    1.02   =   1,101.6. 

CM  =  ^X^  X  K  =  MOO  X  47,100x2,400  = 

PX&  5  X  (i,ioi.6)2 

Next  larger  B.  &  S.  wire 

=  No.  3  =  52,633  CM. 
Loss  of  delivered  power  using  No.  4  wire 

_  2,500  X  47,100  X  2,400  _ 

52,633  X  (1,101.6) 
Total  volts  lost  in  line 

-f  X  E  4.4  X  1,101.6  X  1.35        , 

=  -        -xM=-  -^2  =  65.5. 

100  100 

Generator  voltage        =  1,101.6  +  65.5  =  1,167.1. 

In  a  60  cycle  single-phase  system,  with  the  same  condi- 
tions as  in  the  above  example,  the  values  will  be  the  same, 
with  the  exception  of  the  volts  lost  in  the  line. 

4.4  x  1,101.6  x  i. ii  ..    .    ,  .    r 

—  =  C7.8  =  volts  lost  in  line. 
100 

1,101.6  +  53.8  =  1,155.4  =  generator  voltage. 


CALCULATION   OF   TRANSMISSION   LINES.      231 

TWO-PHASE    SYSTEM.  -  60    CYCLES.       FOUR-WIRE 
TRANSMISSION. 

EXAMPLE  :  2,500  H.  P.  delivered,  5  miles,  at  secondaries 
of  step-down  transformers.  Pressure  between  lines  at  re- 
ceiving end,  6,000  volts.  Energy  -loss  in  line  and  in  step- 
down  transformers  (no  step-up  transformers),  10  per  cent 
of  delivered  power.  Efficiency  of  transformers,  97.5  per 
cent.  Power  factor  of  load,  80  per  cent.  Find  size  of 
conductors  and  voltage  drop  in  transmission  line. 
Power  delivered  at  step-down  secondaries. 

=  ^^  =  2,564  H.P.  =  1,912.7  K.W. 

*s  I  O 

Energy  loss  in  line  =  7.5  per  cent. 
5,280  x  <;  X  1,012,700 

CM-  =          7.5  X  (6.000)'      -  X  I'69°  =  3I5'94°  CM- 
Three  No.  o  B.  &  S.  wires  have  an  area  of  316,776  C.M. 
The  energy  loss,  using  3  of  this  size  in  parallel,  making  a 
total  of  1  2  No.  o  B.  &  S.  wires  in  all,  is  : 
5,280  X  5  X  1,912, 


x  1,690  =  7.48  per  cent. 
316,776  x  (6,ooo)2 

Power  lost  in  line 

=  2,564  x  .0748  =  195.8  H.P. 
Volts  lost  in  line 

P  X  E  7.48  X  6,000  X  1.28 

=  _        -  x  M=^  -  =574- 

100  100 

.*.  Generator  voltage  =  6,574. 

Current  in  line 

W  1,012,700 

=  —  x  T  =     \         -  X  .625  =  199  amperes. 
E  6,000 

The  current  is,  in  fact,  slightly  greater,  as  no  account 
has  been  taken  of  the  hysteresis  current  in  the  trans- 
formers. This  will  increase  the  above  result  about  i-J-  per 
cent. 


232       POLYPHASE  APPARATUS   AND   SYSTEMS. 

THREE-PHASE    SYSTEM.  -  6O    CYCLES.       THREE-WIRE 

TRANSMISSION. 

EXAMPLE  :  Same  conditions  as  preceding.     Find  size  of 
conductors  and  voltage  drop  in  transmission  lines. 

Power  delivered  to  transformers 

=  2,564  H.P.  =  1,912.7  K.W. 


Energy  loss  in  line  =  7^  per  cent. 

5,280  X  5  X  1,912,700 

CM.  =  —  —  -^  —  x  1,690  ==  3iS>94°  C.M. 

7.5  X  (6,oooy 

Three  No.  o  B.  &  S.  wires  have  an  area  of  316,776  C.M. 
For  the  three  branches  of  the  three-phase  system  9  wires 
will  be  required. 

£,280  x  t;  X  1,012.700 

Energy  loss  is  =  -  —  -—  -  x  1,690  =  7.48  per  cent. 

316,776  x  (6,ooo)2 

Power  loss  in  line 

=  2,564  X  .0748  =  195.8  H.P. 
Voltage  drop  in  line 

7.48  x  6,000  X  1.28 


100 


=  574- 


.•.  Generator  voltage  =  6,574. 
Current  in  line 

1,912,700 
6,000 

The  hysteresis  current  will  increase  this  result  by  about 
i^  per  cent. 

THREE-PHASE    SYSTEM.  60    CYCLES.       FOUR-WIRE 

SECONDARY. 

EXAMPLE  :  Required,  the  size  of  conductors  from  trans- 
formers to  the  distributing  centre  of  a  four-wire  secondary 
system  for  lights  and  motors.  The  load  consists  of  four 


CALCULATION   OF  TRANSMISSION   LINES.     233 

1 6  H.P.,  200  volt  induction  motors,  and  750  half -ampere 
1 5  c.p.,  1 1 5  volt  lamps.  Length  of  secondary  wiring  from 
transformers  to  distribution  centre,  600  feet.  About  15 
volts  drop  on  lighting  circuits  from  transformers  to  distrib- 
uting centre.  Efficiency  of  motors,  85  per  cent.  Five 
volts  drop  on  circuits  from  distributing  centre  to  motors. 
Voltage  at  distributing  point  between  main  lines  is  205. 
Current  in  main  lines  for  motors  is 

4  X  15  X  746  x  .725 

—  =  IQI  amperes. 

.85    X    200 

Current  for  transformers  from  lamps  is 

(75°  X  .5  X  115)  X  -607 

-  =  131  amperes. 
200 

Total  current  from  transformers  is 

131  4-  I9I  =  322  amperes. 
For  motors, 

W 

191  = .725.      W  =  54,000. 

For  lamps, 

131  x  -  -  X  .607.      W '=  44,240.     Total  watts  =  98,240. 

Taking  for  trial  two  No.  o  B.  &  S.  wires  in  parallel  for 
each  of  the  main  conductors  as  preferable  to  one  No.  oooo, 

then 

ooo  x  98,240   ^ 


2  X  105,592  X  2053 

1,200'  x  44,240  4-  1,690  x  54,000  _ 

98,240 
Volts  loss  in  lines 

=  9-75  X  205  X  1.32  =  26  ^ 

100 

Volts  at  transformers  between  main  lines  =  231.4. 
Actual  drop  between  main  conductors  and  neutral  to  distrib- 
uting point 

=  26.4  X  —  =  15-2  volts. 
200 


234       POLYPHASE  APPARATUS   AND   SYSTEMS. 

The  section  of  the  neutral  conductor  should  be  about 
131  x  2  x  105,592 


322 
B.  &  S.  wire  with  a  section  of  83,694  C.M.  for  the  neutral. 

MONOCYCLIC     SYSTEM.  -  60    CYCLES.       MOTOR    AND     LIGHTS 
ON    SEPARATE    TRANSFORMERS. 

EXAMPLE  :  1,500  half  ampere  104  volt  lamps.  One 
25  H.P.  110  volt  induction  motor;  efficiency,  85  per  cent. 
Distance  from  generator  to  transformer,  3,000  feet.  Dis- 
tance from  transformers  to  motor,  100  feet.  Loss  in  motor 
circuit,  2-J-  per  cent.  Loss  of  energy  in  transformers,  3  per 
cent.  Loss  in  primary  circuit,  4  per  cent.  Generator 
voltage,  1,040  at  no  load. 

2c  x  746 

Input  at  motor  =  —  —  =  21,940  watts. 

•85 

100  X  21,940 

C.M.  =  -  ?—-  x  3,380  —  2415,000  No.  oooo 

2.5  X   no' 

B.  &  S.  wire  =  211,600  C.M.,  but  as  two  No.  o  B.  &  S.  wires 
will  give  the  same  loss,  and     '       =  69.2   per  cent  as  great  a 

drop  in  voltage,  they  are  preferable.     Making  each  motor  lead 
of  two  No.  o  B.  &  S.  wires  in  parallel,  then 

100  X  21,040  X  3,^80 

P=-  ^  ,    =2.9  per  cent. 

105,592   X  2  X  no" 

Volts  loss  to  motors 

_  2.9  x  no  X  1.28  _ 

100 

Volts  at  primaries  of  transformers  for  motors 

=  1.05  X  9  X  (no  4-  4)  =  1,076. 
Volts  on  secondaries  of  lighting  transformers 

1,076 

=  -  -  =  104.1;. 

1.03  X  10 


CALCULATION    OF   TRANSMISSION    LINES.      235 

Watts  at  primaries  of  motor  transformers 
21,940  x  1.029 


Watts  at  primaries  of  lighting  transformers 
=  '-500  X  .5   X   104.5 

•97 
Total  watts  delivered  at  transformers 

=  23,200  +  80,800  =  104,000. 
Power  factor  of  load  is 

23,200  x  .80  -h  80,000  x  .95 

-  =  .QI 

104,000 
r       2,160 

K  =  --  5-   =    2,6lO. 

•91 
C'M'  =  3X°0  *  2'6l°  ~  '75,500. 


Taking  No.  ooo  B.  &  S.  wire  X  167,805  C.M.,  then 

^,000  X  104,000 

P=  -  —^  X  2,610  =  4.19  per  cent. 

167,805  x  i,o762 

Drop  in  primary  circuit 

_  419  x  1,076       1.49  X  80.8  -|-  1.62  X  23.2 
100  104 

=  68.5  volts. 
Voltage  between  outside  lines  at  generator 

=  1,076  -f-  68.5  =  1,144.5  v°lts. 
Current  in  main  conductors 

104,000 

=  —  -  =  1  06.  i  amperes. 

1,076  X  .91 

Primary  teaser  wire 

_     3>          x  j6*  g0(-  _  37,400  C.M.  required. 
104,000 

Use  No.  4  B.  &  S.  wire  with  a  section  of  41,742  C.M. 

Graphical  Illustration  ---  The  curves  on  pages  236-239, 
Figs.  157,  158,  and  159,  have  been  calculated  from  the 
preceding  formula  and  table  of  constants  relating  to  the 
three-phase  system  only.  They  will  be  found  useful  for 


236       POLYPHASE   APPARATUS   AND   SYSTEMS. 


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CALCULATION   OF   TRANSMISSION   LINES.      237 


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238       POLYPHASE   APPARATUS  AND    SYSTEMS. 


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CALCULATION   OF   TRANSMISSION   LINES.      239 

calculating  and  approximately  determining  the  copper  re- 
quired for  transmitting  any  amount  of  power  any  distance 
at  voltages  varying  from  1,000  to  15,00x3. 

For  cases  that  fall  outside  the  limits  of  the  curves,  the 
size  of  the  wire  may  be  found  by  applying  the  following 
rules : 

With  given  power  delivered,  line  loss,  and  voltage,  the  cross- 
section  of  the  conductor  will  vary  directly  as  the  distance. 

With  given  distance  of  transmission,  line  loss,  and  voltage, 
the  cross-section  of  the  conductor  will  vary  directly  as  the  power 
delivered. 

With  given  distance  of  transmission,  power  delivered,  and 
voltage,  the  cross-section  of  the  conductor  will  vary  inversely 
as  the  loss  of  energy  in  the  line. 

With  given  distance,  power  delivered,  and  line  loss,  the  cross- 
section  of  the  conductor  will  vary  inversely  as  the  square  of 
the  voltage. 

The  voltages  are  taken  as  those  at  the  receiving  end. 
The  line  loss  has  been  assumed  to  be  10  per  cent  of  the 
delivered  energy.  In  plotting  the  curves  the  following 
power  factors  have  been  assumed  : 

For  lighting  load 95% 

For  mixed  load  of  induction  motors  and  lights   .     .     85  °/0 
For  induction  motor  load     ..     *. 80% 

To  illustrate  the  use  of  the  curves,  find  the  size  of  the 
wire  required  to  transmit  5,100  H.P.,  to  be  used  for  incan- 
descent lighting,  a  distance  of  five  miles,  the  current  loss 
being  10  per  cent,  and  the  pressure  at  the  primaries  of 
step-down  transformers,  10,000  volts.  The  curve  (Fig. 
157)  shows  that  each  of  the  three  wires  must  have  a  cross- 
section  of  1 20,000  circular  mils.  If  the  power  delivered  is 
to  be  consumed  by  induction  motors,  other  conditions  re- 


240      POLYPHASE   APPARATUS   AND    SYSTEMS. 

maining  the  same,  the  conductor  must  have  a  cross-section 
equivalent  to  170,000  circular  mils  each,  or  slightly  larger 
than  No.  ooo  wire.  Or,  supposing  the  wire  to  have  been 
strung  on  the  assumption  that  lights  would  be  supplied, 
the  line  loss  and  pressure  being  the  same  as  above,  it  will 
be  seen  that,  if  the  load  is  changed  to  induction  motors, 
only  3,600  H.P.  will  be  delivered  from  these  lines.  This  is  a 
striking  illustration  of  the  decrease  in  the  carrying  capacity 
of  the  line,  due  to  low  power-factors,  which  load  the  line, 
and  the  generators  as  well,  with  so-called  wattless  current. 

If  the  distance  is  raised  to  ten  miles,  the  size  of  wire 
required  for  the  same  transmission  is  doubled  in  both  the 
above  examples.  If  the  distance  is  increased  to  ten  miles, 
and  the  energy  loss  reduced  to  five  per  cent,  the  cross- 
section  of  conductor  will  have  to  be  made  four  times  as 
great. 

Three  wires  of  about  No.  3  size  will  transmit  a  lighting 
load  of  5,100  H.P.  a  distance  of  five  miles,  the  pressure 
being  15,000  at  the  receiving  end.  It  will  take  three  con- 
ductors of  cross-section  corresponding  to  a  size  between 
No.  i  and  No.  2  to  transmit  the  same  power  for  induction 
motor  use,  and  three  No.  2  wires  to  transmit  the  same 
energy  for  a  mixed  load  of  lights  and  motors. 

For  determining  the  size  of  transmission  lines  with  vol- 
tages of  5,000  and  less,  the  curves  in  Fig.  158  will  be 
found  most  convenient. 

Fig.  159  represents  the  curves  of  percentage  drop  of 
voltage  in  transmission  lines,  at  varying  frequencies  and 
power  factors.  The  curves  show  the  values  of  the  con- 
stant, Mt  plotted  from  the  table  on  page  238,  and  are  based 
on  10  per  cent  energy  loss  in  line. 

/    study  of   the  curves  shows   some  interesting  facts. 


CALCULATION   OF  TRANSMISSION   LINES.      241 

When  the  transmission  is  effected  at  30  cycles,  it  will  be 
noticed  that,  for  all  commercial  sizes  of  wires,  the  voltage 
drop  is  less  with  a  load  of  low  power-factor  than  with 
one  of  high  power-factor.  For  illustration,  assume  that 
the  transmission  requires  conductors  of  120,000  circular 
mils  each,  the  energy  loss  being  10  per  cent  of  the  deliv- 
ered power.  At  30  cycles,  the  drop  in  voltage  is  10.1 
per  cent  when  the  power  factor  is  80  per  cent,  10.4  per 
cent  when  the  power  factor  is  85  per  cent,  and  11.3  per 
cent  when  the  power  factor  is  95  per  cent.  On  the  other 
hand,  the  same  transmission -at  125  cycles  shows  a  higher 
voltage  drop  with  low  power-factors.  The  voltage  drops 
18.3  per  cent  with  a  95  per  cent  power-factor,  21.2  per 
cent  with  an  85  per  cent  power-factor,  and  21.7  per  cent 
when  the  power  factor  is  80  per  cent. 

A  curious  condition  exists  at  60  cycles.  The  voltage 
drop  is  less  with  a  power  factor  of  95  per  cent,  than  when 
the  power  factor  is  85  per  cent ;  but  an  80  per  cent 
power-factor  gives  a  drop  approximately  the  same  as  that 
due  to  a  power  factor  of  95  per  cent.  The  curves  also 
graphically  illustrate  the  reduction  in  voltage  drop  to  be 
gained  by  subdividing  the  conductors.  A  No.  oo  wire, 
used  in  a  60  cycle  transmission  of  power  for  induction 
motors,  shows  a  drop  of  14.3  per  cent.  By  subdividing 
the  wire  into  two  No.  2  wires,  and  equivalent  cross-sec- 
tion, the  voltage  drop  is  reduced  to  10.6  per  cent. 

It  will  b-?  seen,  from  the  curves,  that,  by  subdividing 
the  conductor  sufficiently,  a  wire  of  a  size  can  be  selected, 
which,  for  all  commercial  power-factors  and  frequencies, 
will  transmit  any  amount  of  power,  with  a  drop  of  voltage 
in  the  line  actually  less  than  the  energy  loss.  This  ap- 
parent anomaly  is  explained  in  Chapter  I.,  under  the  para- 
graph, "  Voltage  Drop  Due  to  Varying-  Power  Factor." 


242   POLYPHASE  APPARATUS  AND  SYSTEMS. 

Resonance  Effect.  —  What  is  known  as  the  resonance 
effect  of  a  circuit  is  the  rise  of  E.M.F.  at  the  far  end,  above 
that  at  the  generator  end.  This  phenomenon  takes  place 
when  the  natural  period  of  discharge  of  a  circuit  is  equal 
to  the  frequency  of  the  generator  E.M.F.  It  is  complete 
when  the  self-induction  and  capacity  exactly  neutralize 
each  other.  The  charging  current  of  the  line,  due  to  the 
capacity,  then  produces  an  E.M.F.  of  self-induction  equal 
to  the  generator  E.M.F. 

In  transmission  lines,  where  the  inductance  and  capacity 
do  not  exactly  neutralize  each  other,  it  is  possible  for  par- 
tial resonance  to  be  present.  The  circuit  can  be  brought 
into  complete  resonance  by  the  addition  of  a  condenser  or 
a  reactance,  according  as  it  lacks  the  proper  amount  of 
either  capacity  or  inductance.  It  is  conceivable  that  an 
unexpected  rise  of  pressure  may  occur  of  sufficient  extent 
to  uestroy  the  insulation  of  line  and  of  apparatus. 

The  rise  of  pressure  due  to  complete  resonance  is  limited 
by  the  ohmic  resistance  of  the  circuit.  For  this  reason, 
and  because  practical  transmissions  of  power  are  accom- 
plished at  a  comparatively  low  frequency,  the  possible  rise 
of  pressure  at  the  receiving  end  is  not  likely  to  be  danger- 
ously high. 

For  very  long  power  transmissions,  where  resonance 
effects  may  be  expected,  it  is  desirable  to  employ  genera- 
tors producing  an  E.M.F.  wave  which  is  sinusoidal.  A 
distorted  wave  of  E.M.F.  of  the  same  period  can  be  resolved 
into  a  number  of  simple  harmonic  components  of  a  higher 
frequency.  These  higher  harmonics  have  the  same  effect 
as  an  E.M.F.  wave  of  the  same  frequency  and  magnitude. 


INDEX. 


Am  blast  transformers,  130. 
Alternating  circuit,  flow  of  cur- 
rent in,  3. 
energy  in,  10. 
Alternations,  16. 
Alternators  (see  Generators). 
Ampere  turns,  rotary  converter, 

112. 

Angle  of  lag,  u. 
Apparent  efficiency,  83. 
energy,  n. 
resistance,  14. 

Arc  lamps,  on  low  frequency  cir- 
cuits, 209. 

Armature,  inductance,  34. 
induction  motors,  60. 
multitooth  construction,  34. 
reaction,  34,  35. 
resistance  of   induction  mo- 
tors, 60. 

unitooth  construction,  33. 
Auto  converters  for  starting  in- 
duction motors,  64. 

BALANCED   three-phase   system, 

187. 
two-phase    system,    171-174, 

176. 
Blowers  for  cooling  transformers, 

132. 

Breakdown  point  induction  mo- 
tors, 75-77. 
synchronous  motors,  93,  94. 


CALCULATION    of    transmission 
lines,  constants  for,  221, 224. 

Capacity,  4. 

and   magnetic   reactance    in 

same  circuit,  8. 
of  transmission  lines,  224. 

Charging  current  in  transmission 
lines,  224,  225. 

Choking  coils   for  lightning   ar- 
resters, 152. 

Coefficient  of  self-induction,  4. 

Combinations  of  circuits  in  poly- 
phase systems,  166. 

Compensators  for  induction  mo- 
tors, 64. 
synchronous  motors,  97. 

Composite  winding  of  generators, 

39- 

Compounding  of  generators,  38. 
Condensance,  8. 
Condenser,  use  of,  with  induction 

motors,  86. 
Conductors     (see     Transmission 

lines). 

Connections  of  polyphase  wind- 
ings, 1 66,  1 80. 
delta,  168,  181,  182. 
interlinked,  167. 
ring,  168. 
star,  167. 
Y,  168,  181,  182. 

Constants  for  line  calculation.  223. 
Converter  (see  Rotary  converter). 


243 


244 


INDEX. 


Cooling  of  transformers  by  air 

blast,  130. 
natural  draft,  134. 
oil,  122. 
water,  127. 

Copper,  amount  of,  required  with 
different     polyphase    sys- 
tems, 214. 
losses  in  transformers,   136, 

137- 

Core  losses  in  transformers,  137. 
Cosine  of  lag  angle,  n. 
Counter  E.M.F.,  3. 
Currents,    alternating,  definition 

of  terms,  i. 

Current,  armature  in  rotary  con- 
verter, 112. 

in  synchronous  motor,  100. 
lagging,  4. 
leading,  5. 
Wattless,  12. 
Curve  of  E.M.F.,  2. 

generator  efficiency,  44. 
induction  motor  efficiency, 

74,  86. 

transformer  efficiency,  136. 
three-phase  E.M.F.,  180. 
two-phase  E.M.F.,  167. 
Curves  of  line  losses,  236,  237. 
voltage  drop  in  transmission 
lines,  238. 

DELTA  connection  of  windings, 

161,  181,  182. 
Distribution  circuits,  monocyclic, 

197. 
three-phase    four-wire,     184, 

185,  190. 

three-phase  three-wire,  176. 
two-phase  four-wire,  173. 
two-phase  three-wire,  176. 


EFFICIENCY  generators,  44. 

ind  uction  motors,  74-86. 

sychronous  motors,  92. 

transformers,  136. 
Electrical  resonance,  242. 
Electromotive  force,  2. 

impressed,  5. 

energy  component  of,  6. 

induction  component  of,  6. 
Electromotive  force,  curve  of,  2. 

three  phase,  180. 

two  phase,  167. 
Energy  apparent,  10. 

current,  12. 

loss  in  circuit,  13. 
Engine,  regulation  for  parallel  op- 
eration of  generators,  48. 
Excitation,  rotary  converters,  1 1 1- 
114. 

synchronous  motor,  99. 
Exciting  current  of  transformers, 

i37. 

Exciter  panel,  140,  146. 

Exciters,  capacities  of,  for  gener- 
ators and  motors,  101. 

FACTOR,  induction,  u. 

power,  ii. 
Farad,  the,  5. 
Field  induction  motor,  68. 
Field  excitation,  generator,  38. 

rotary  converter,  111-113. 

rotary   converter,    effect    on 
voltage,  no. 

synchronous  motor,  99. 
Flux,  magnetic,  3. 
Frequency  changes,  163. 

choice  of,  207,  212. 

definition,  16. 

effect  of  on  parallel,  81. 

operation  of  generator,  209. 

high,  207. 


INDEX. 


245 


Frequency,  induction  motors,  81. 
limit    of    rotary    converters, 

H3- 
low,  209. 

GENERATOR,  armature  construc- 
tion, 20,  33. 

armature  inductance,  35. 

armature  reaction,  35. 

armature  windings,  33. 
Generators, 

conditions  effecting  cost  of, 

57- 

efficiency,  43. 
electro-motive  force,  34. 
elementary  forms,  17. 
field  excitation,  34. 
inductor  type,  27. 
losses,  43. 

methods  of  driving,  50. 
monocyclic  windings,  196. 
parallel  running,  46. 
revolving  armature  type,  18. 
revolving  field  type,  21. 
speed,  45. 
speed    regulation   of  engine 

for  driving,  48. 
three-phase    windings,    180- 

182. 

two-phase  windings,  166-168. 
Geographical  illustrations  of  line 

losses,  236,  237. 
voltage  drops,  238. 
Grounding  of  lightning  arresters, 

154. 

HARMONIC  motion,  simple,  2. 

Henry,  the,  4. 

High  voltage  generators,  33. 

IDLE  currents  (see  Wattless  cur- 
rent). 


Impedance,  7. 
Impressed  E.M.F.,  5. 
Inductance,  3. 
Induction,  4. 

compound  E.M.F.,  6. 

factor,  ii. 
Induction  motors,  60. 

condensers  for,  86. 

construction  of  primary  and 
secondary,  68. 

efficiency,  83. 

frequency,  81. 

initial  voltages,  83. 

low  inductance  type,  77. 

methods  of  starting,  61. 

monocyclic,  203. 

principles  of  operation,  60. 

power  factor,  83. 

single  phase,  87. 

speed  regulation,  78. 

starting  torque  and  current, 
72. 

transformer  capacities  for,  84. 

variable  armature  resistance 
type,  62,  67,  76. 

voltage,  82. 

wiring  for,  83. 

with     short-circuited     arma- 
tures, 63,  67,  76.' 
Inductive  loads,  83-100. 
Inductor  generator,  27. 
Insulators,  156. 

glass,  156,  158. 

porcelain,  156. 

provo  type,  158. 
Iron  losses,  generators,  43. 

transformers,  137. 

LAG,  angle  of,  n. 
Lightning  arresters,  149. 

G.  E.  type,  151. 

installation  of,  152. 


246 


INDEX. 


Lightning    arresters,    protection, 

146. 

Wurtz  type,  150. 
Line  (see  Transmission  lines). 
Line  constants  for  power  trans- 
mission, 224. 

protection  from  lightning  ef- 
fects, 149. 
Lines  of  force,  3. 
Load,  maximum  induction  motor, 

76,  77- 

synchronous  motor,  93. 
Longdistance  power  transmission 
by     three  -  phase     system, 
190. 

by  two-phase  system,  170-173. 
Losses  in  generators,  43. 
induction  motors,  85. 
transformers,  136. 

MAGNETIC  circuit,  inductor  gen- 
erator, 28. 

revolving  field  generator,  23. 
Magnetic  field  induction  motor,6o. 
Magnetizing  current,  75. 
Measurement  of  power  in  mono- 
cyclic  circuits,  204. 
three-phase  circuits,  186,  187. 
two-phase  circuits,  176. 
Mesh  connection  (see  Ring  con- 
nection). 
Monocyclic  system,  194. 

distributing  circuits,  197. 
features  of,  194. 
generator    armature    connec- 
tions, 196. 

measurement  of  power  in,  204. 
motors  for,  203. 
transformation       to      three- 
phase,    201-203. 
transformer  connections  for 
motors  and  lights,  200. 


Motor  connections  in  three-phase 

system,  184,  185. 
two-phase   system,   170,    171, 

177- 

Motor  generators,  165. 
Multiphase  (see  Polyphase). 

NEUTRAL    point   in  three-phase 
system,  180,  182,  185,  187. 

Ohm's  law,  modification  of,  in  al- 
ternating current  circuits,  3. 

Oiled  cooled  transformers,  122. 

Oscillatory  character  of  lightning 
discharges,  152. 

Output    maximum   of    induction 

motors,  76,  77. 
synchronous  motors,  93. 

PARALLEL    running    of    genera- 
tors, 46. 

Periodicity  (see  Frequency). 
Phase  displacement  (see  Angle  of 

lag). 

Phase  transformation,  171. 
Polycyclic  System,  193. 
Polyphase  circuits,  various  con- 
nections of  (see  Two-phase, 
Three-phase,    and     Mono- 
cyclic  systems), 
currents,  166. 
systems    and    combinations, 

1 66. 

transformers,  120. 
Power  factor,  1 1 . 

induction  motors,  83. 
rotary  converters,  116. 
synchronous  motors,  102. 
voltage  drop  due  to,  13. 
Power  measurement,  monocyclic 

system,  204. 

three-phase  system,  187,  188. 
two-phase  system,  176. 


INDEX. 


247 


Power  transmission,  longdistance 
by  three-phase  system,  190. 
two-phase  system,  170,  173. 
Primary  of  induction  motor,  60- 

68. 

Prime  movers  for  driving  genera- 
tors, 50. 

Pressure  Regulators,  160. 
polyphase  type,  160. 
single-phase  type,  160. 
Still  well  type,  161. 
Punchings,   generator   armature, 
33- 

RADIATING  surface  of  transform- 
ers, 121. 

Ratio  of  transformation  of  rotary 
converters,  no. 

Reactance,  7. 

of   transmission  conductors, 
224. 

Reaction,    generator    armatures, 

35- 

Rectifiers,  162. 

Regulation,  inherent  of  genera- 
tors, 42. 

of  transformers,  138. 
speed,  of  induction  motors, 

78. 
of  synchronous  motors,  92, 

93- 
Resistance  apparent,  14. 

copper  conductors,  224. 

virtual,  9. 

Resonance  effect,  242. 
Reversing  induction  motors,  68. 
Ring  winding,  168. 
Rotary  converters,  106. 

armature  connections,  107. 

armature  reaction,  113. 

general  features,  106. 

limit  of  frequency,  113. 


Rotary  converters  made  from 
direct  current  generators, 
106. 

parallel  operation,  119. 
power  factor,  116. 
ratio  alternating  to  direct  cur- 
rent voltage,  no. 
starting  and  running,  118. 
types  and  uses,  in. 
voltage  variation,  114,  115. 
Rotor  of  induction  motor,  60-68. 

SECONDARY  systems  of  distribu- 
tion, monocyclic,  197. 
three-phase    four-wire,    183- 

185. 
three-phase  three-wire,    181- 

184. 

two-phase  four-wire,  173. 
two-phase  three-wire,  176. 
Self-induction,  coefficient  of,  4. 
Simple  harmonic  motion,  2. 
Sine  wave,  2. 
Single-phase    induction    motors, 

87. 

synchronous  motors,  93. 
wiring,  98. 
Skin  effect,  10. 
Slip  of  induction  motors,  61. 
Speed  control  of  induction  mo- 
tors, 79. 

effect  of,  on  cost  of  genera- 
tors, 57. 

regulation  of  engines  for  par- 
allel running,  48. 
variation    of    induction    mo- 
tors, 78. 
Star  connection  of  windings,  168, 

181. 

Starting   current   induction    mo- 
tors, 61,  72,  73. 
synchronous  motors,  95. 


248 


INDEX. 


Starting  of  induction  motors,  61. 
rotary  converters,  118. 
synchronous  motors,  95. 
Starting  torque  effect  of  voltage 

on,  75,  76,  94. 

of  induction  motors,  72,  73. 
of  synchronous  motors,  93. 
Static      transformers      (see 

Transformers). 
Synchronizing  devices,  154. 
Synchronous  motors,  92. 
advantages  of,  92. 
field  excitation,  99. 
methods  of  starting,  95. 
monocyclic,  94. 
power  factor,  102. 
speed,  92. 

torque  and  output,  93. 
used  as  condensers,  104. 
voltage,  94. 
Temperature     of     transformers, 

122. 
Theory  of    action   of    induction 

motors,  60. 
Three-phase  circuits   for   power 

distribution,  190. 
curves  of  E.M.F.,  180. 
four-wire  system,  185. 
long    distance     transmission 

circuits,  192. 
motor  connections,  184. 
three- wire  system,  199. 
transformer  connections,  181. 
Three-phase  system,  180. 

measurement    of    power    in, 

186. 
Torque    diagram    of     induction 

motors.  73. 
starting  of  induction  motors, 

72. 

starting  of  synchronous  mo- 
tors, 93. 


Transformation  of  phases,  171. 
Transformer  connections,  mono- 
cyclic,  200. 

three  phase,  181. 

two  phase,  170. 
Transformers,  120. 

air  blast  type,  130. 

efficiency,  135. 

losses,  135. 

natural  draft  type,  134. 
Transformers,   operation    of    air 
blast,  132. 

polyphase,  120. 

regulation,  138. 

self-cooled  oil  type,  122. 

water-cooled  oil  type,  127. 
Transmission    lines,    calculation 
of,  221. 

capacity  of,  224,  225. 

charging  current  in,  224,  225. 

inductance  of,  224,  225. 

resistance  of,  224,  225. 

voltage  drop  in.  221,  226,  238. 
Two-phase  four-wire  system,  173. 

generator    armature   connec- 
tions, 1 68. 

interlinked  windings,  167. 

separate  windings,  167. 

three-wire  system,  176. 

to  three-phase,  171. 

transformer  connections,  170. 

unbalancing,  177. 
Two-phase  system,  166. 

relations  of  E.M.F.  in,  166. 

UNIT  of  capacity,  5. 

of  self -inductance,  4. 

VOLTAGE   drop   in   transmission 

lines,  221,  226,  228. 
Voltage,  effects  of  on  output  of 

induction  motor,  82. 


INDEX. 


249 


Voltage  of  synchronous  motor,  94. 

Voltage  of  induction  motor,  no. 
relation  of    line  to  induced 
E.M.F.  in  three-phase  gen- 
erators, 36. 

WATER  wheels  as  prime  movers, 

47,  5i,  54- 

Wattless  current,  12. 
Wattless  or  magnetizing  current 

in  induction  motor,  85. 
transformers,  139. 
Wattmeter  for  measuring  power, 
in  monocyclic  circuits,  204. 
three-phase  circuits,  187. 
two-phase  circuits,  176. 


Watts  apparent,  n. 

Windings  generator  armature,  33. 

interlinked,  167. 

monocyclic,  196. 

three  phase,  180-183. 

transformers,  166. 

two  phase,  167. 

primary  of  induction  motor, 
68. 

secondary  of  induction  mo- 
tor, 68. 
Wiring  formulas,  227. 

application  of,  230. 

Y  connection  in  three-phase  sys- 
tem, 181,  182,  186. 


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